Birgitte Freiesleben de Blasio

• Kandula, Sasikiran; Rø, Gunnar Øyvind Isaksson; Leblanc, Marissa Erin & Blasio, Birgitte Freiesleben de (2026). Categorical short-term forecasts of respiratory pathogens in Europe. Epidemics. ISSN 1755-4365. 56, p. 1–9. doi: 10.1016/j.epidem.2026.100938. Full text in Research Archive
• Palomares, Alfonso Diz-Lois; Blasio, Birgitte Freiesleben De; Chan, Louis Yat Hin; Ruscio, Francesco Di & Midtbø, Jørgen E. (2026). Combining an agent-based model with Gaussian process emulation to model the emergence of the SARS-CoV-2 Omicron variant in Norway. Epidemics. ISSN 1755-4365. 54. doi: 10.1016/j.epidem.2026.100895. Full text in Research Archive
• Rø, Gunnar Øyvind Isaksson; Lyngstad, Trude Marie; Seppälä, Elina Marjukka; Skodvin, Siri Nærland; Trogstad, Lill & White, Richard Aubrey [Show all 13 contributors for this article] (2025). Estimating the trend of COVID-19 in Norway by combining multiple surveillance indicators. PLOS ONE. 20(1). doi: 10.1371/journal.pone.0317105. Full text in Research Archive Show summary

  Estimating the trend of new infections was crucial for monitoring risk and for evaluating strategies and interventions during the COVID-19 pandemic. The pandemic revealed the utility of new data sources and highlighted challenges in interpreting surveillance indicators when changes in disease severity, testing practices or reporting occur. Our study aims to estimate the underlying trend in new COVID-19 infections by combining estimates of growth rates from all available surveillance indicators in Norway. We estimated growth rates by using a negative binomial regression method and aligned the growth rates in time to hospital admissions by maximising correlations. Using a meta-analysis framework, we calculated overall growth rates and reproduction numbers including assessments of the heterogeneity between indicators. We find that the estimated growth rates reached a maximum of 25% per day in March 2020, but afterwards they were between -10% and 10% per day. The correlations between the growth rates estimated from different indicators were between 0.5 and 1.0. Growth rates from indicators based on wastewater, panel and cohort data can give up to 14 days earlier signals of trends compared to hospital admissions, while indicators based on positive lab tests can give signals up to 7 days earlier. Combining estimates of growth rates from multiple surveillance indicators provides a useful description of the COVID-19 pandemic in Norway. This is a powerful technique for a holistic understanding of the trends of new COVID-19 infections and the technique can easily be adapted to new data sources and situations.

• Kandula, Sasikiran; Kristoffersen, Anja Bråthen; Rø, Gunnar Øyvind Isaksson; Leblanc, Marissa Erin & Blasio, Birgitte Freiesleben de (2025). A multi-model approach to estimate excess mortality in the Nordics, 2020–2023. Public Health. ISSN 0033-3506. 242, p. 131–138. doi: 10.1016/j.puhe.2025.02.032. Full text in Research Archive Show summary

  Objectives Excess mortality has been one of the commonly used measures of the population health effects of the COVID-19 pandemic. Denmark, Finland, Norway and Sweden share several health and socioeconomic characteristics but adopted different control measures and experienced varying degrees of case and hospitalization burden during the pandemic. Using mortality trends between 2001 and 2019 and a combination of models, we estimated and compared annual and monthly excess mortality in these countries nationally as well as stratified by age, sex and subnational regions between 2020 and 2023. Study design Multi-model study. Methods Three methods were used to estimate mortality: i) a Bayesian spatial model with a random effect component for spatial dependence among subregions and trend and seasonality terms; ii) a Bayesian GAMM model with terms for annual trend (a thin-plate spline) and within-year seasonality (a cyclic cubic spline); and, iii) a combination of autoregressive and exponential trend smoothing methods. Estimates from these approaches were combined using model averaging. Results Based on age-standardized mortality rates (per 100,000 population) of the averaged estimates: Finland had the highest cumulative excess mortality of the four countries; older age groups (70+ year) accounted for nearly all excess mortality; men had higher excess rates than women; and capital regions had some of the lowest rates relative to other regions in each country. With a few exceptions, mortality in 2023 returned to pre-pandemic levels. Model verification indicated good calibration and superior skill of the combination model relative to its component models. Conclusions We believe our approach better quantifies uncertainty than individual models, and our estimates are comprehensive, spatially, temporally and demographically well-resolved, and can support further association studies.

• Kandula, Sasikiran; Blasio, Birgitte Freiesleben de & Leblanc, Marissa Erin (2025). Real-time monitoring of excess mortality under a new endemic regime. Eurosurveillance. ISSN 1025-496X. 30(25/26), p. 1–12. doi: 10.2807/1560-7917.ES.2025.30.25.2400753. Full text in Research Archive Show summary

  BACKGROUND Monitoring of mortality to identify trends and detect deviations from normal levels is an essential part of routine surveillance. In many European countries, disruptions in mortality patterns from the COVID-19 pandemic have required revisions to expected mortality estimates (and models) in the current endemic phase of SARS-CoV-2. AIM To identify essential characteristics for future mortality surveillance and describe two Bayesian methods that satisfy these criteria while being robust to past periods of high COVID-19 mortality. We demonstrate their application in 19 European countries and subnational estimates in the United States, and report measures of model calibration. METHODS We used a generalised additive model (GAM) with smoothed spline terms for annual trend and within-year seasonality and a generalised linear model (GLM) with a Serfling component for within-year seasonality and breakpoints to detect trend changes in trend. Both approaches modelled change in population size and group-specific (age and sex) mortality patterns. RESULTS Models were well-calibrated and able to estimate national and group-specific mortality before and during the acute COVID-19 pandemic phase. The effect of inclusion of mortality from the acute pandemic period was primarily an increase in uncertainty in expected mortality over the projection period. The GAM approach had better calibration and less variability in bias among countries. CONCLUSION Models that can adapt to mortality anomalies seen during the acute COVID-19 pandemic period without a need for adjustments to observational data, or tailoring of model specifications, are feasible. The proposed methods can complement operational national and inter-agency surveillance systems currently used in Europe.

• Chan, Yat Hin; Rø, Gunnar; Midtbø, Jørgen E.; Ruscio, Francesco Di; Watle, Sara Sofie Viksmoen & Juvet, Lene Kristine [Show all 19 contributors for this article] (2024). Modeling geographic vaccination strategies for COVID-19 in Norway. PLoS Computational Biology. ISSN 1553-734X. 20(1). doi: 10.1371/journal.pcbi.1011426. Full text in Research Archive Show summary

  Vaccination was a key intervention in controlling the COVID-19 pandemic globally. In early 2021, Norway faced significant regional variations in COVID-19 incidence and prevalence, with large differences in population density, necessitating efficient vaccine allocation to reduce infections and severe outcomes. This study explored alternative vaccination strategies to minimize health outcomes (infections, hospitalizations, ICU admissions, deaths) by varying regions prioritized, extra doses prioritized, and implementation start time. Using two models (individual-based and meta-population), we simulated COVID-19 transmission during the primary vaccination period in Norway, covering the first 7 months of 2021. We investigated alternative strategies to allocate more vaccine doses to regions with a higher force of infection. We also examined the robustness of our results and highlighted potential structural differences between the two models. Our findings suggest that early vaccine prioritization could reduce COVID-19 related health outcomes by 8% to 20% compared to a baseline strategy without geographic prioritization. For minimizing infections, hospitalizations, or ICU admissions, the best strategy was to initially allocate all available vaccine doses to fewer high-risk municipalities, comprising approximately one-fourth of the population. For minimizing deaths, a moderate level of geographic prioritization, with approximately one-third of the population receiving doubled doses, gave the best outcomes by balancing the trade-off between vaccinating younger people in high-risk areas and older people in low-risk areas. The actual strategy implemented in Norway was a two-step moderate level aimed at maintaining the balance and ensuring ethical considerations and public trust. However, it did not offer significant advantages over the baseline strategy without geographic prioritization. Earlier implementation of geographic prioritization could have more effectively addressed the main wave of infections, substantially reducing the national burden of the pandemic.

• Fernandez, Liliana Vazquez; Palomares, Alfonso Diz-Lois; Cabrera, Ana M. Vicedo; Blasio, Birgitte Freiesleben De; Ruscio, Francesco Di & Wisløff, Torbjørn [Show all 7 contributors for this article] (2024). Short-term association between air temperature and mortality in seven cities in Norway: A time series analysis. Scandinavian Journal of Public Health. ISSN 1403-4948. doi: 10.1177/14034948241233359. Full text in Research Archive
• Kandula, Sasikiran; Kristoffersen, Anja Bråthen; Rø, Gunnar Øyvind Isaksson; Leblanc, Marissa Erin & Blasio, Birgitte Freiesleben de (2024). Spatial and demographic heterogeneity in excess mortality in the United States, 2020-2023: a multi-model approach. American Journal of Epidemiology. ISSN 0002-9262. doi: 10.1093/aje/kwae422. Full text in Research Archive Show summary

  In this study, we assessed the overall impact of the Covid-19 pandemic in the United States between 2020 and 2023 through estimates of excess all-cause mortality. Monthly mortality rates over a 19-year period, stratified by age, sex and state of residence were used to forecast expected mortality for the pandemic years. A combination of models – two timeseries, a spatial random effects and a generalized additive -- was used to better capture uncertainty. Results indicate that US national excess mortality decreased in 2023 to 157 thousand (95% prediction interval: 35K-282K) from 502K (436K-567K), 574K(484K-666K) and 377K (264K-484K) during the years 2020-2022, respectively. Unlike in previous years, deaths with Covid-19 as the underlying-cause-of-death possibly accounted for all excess deaths during 2023. While for the older age groups (75+ years) the year 2020, before vaccines were available, had the highest excess mortality rate, the two younger age groups had the highest excess mortality in 2021. In each age group, women were estimated to have consistently lower excess mortality than men. West Virginia had the highest age-standardized excess mortality among all states in 2021 and 2022. Our findings demonstrate the value of a multi-model approach in capturing heterogeneity in excess mortality.

• Fernandez, Liliana Vazquez; Palomares, Alfonso Diz-Lois; Vicedo-Cabrera, Ana Maria; Gasparrini, Antonio; Blasio, Birgitte Freiesleben De & Ruscio, Francesco Di [Show all 9 contributors for this article] (2024). Municipality assessment of temperature-related mortality risks in Norway. Environmental Research. ISSN 0013-9351. 266. doi: 10.1016/j.envres.2024.120614. Full text in Research Archive Show summary

  Background & aim Understanding local vulnerability to heat and cold is crucial for public health planning, yet few studies have provided a nationwide analysis of temperature-related mortality across diverse communities. This study analyses the association between ambient air temperature and non-accidental mortality across mainland Norway, using a constrained hierarchical clustering algorithm to group municipalities with similar geographic, environmental, socioeconomic, and demographic patterns. Methods This study analysed the association between ambient air temperature and non-accidental mortality across 356 Norwegian municipalities, using daily data from 1996 to 2018. We applied a case time series design with distributed lag non-linear models. A downscaling procedure assessed the effect of 21 vulnerability factors on temperature-related mortality risks, using Principal Components Analysis to explore heterogeneity across clusters. Findings Cold temperatures contributed to an estimated 3879 deaths per year (95% CI 3718-4130), while heat was associated with 44 deaths annually (95%CI: 29–58). The highest heat-related mortality risk occurred in the South-East, and the highest cold-related risk in the Central-East. Greater heat-related mortality correlated with medium-to sparsely-populated areas, while higher education levels were linked to reduced vulnerability to both heat and cold. Interpretation By providing the first comprehensive assessment of temperature-related excess mortality and associated risk factors in Norway, our findings underscore the need for targeted, equitable health policies that integrate environmental and socioeconomic factors. These insights are essential to guide climate adaptation strategies, prioritising vulnerable rural communities and socioeconomically disadvantaged groups to mitigate future climate-related health impacts.

• Veneti, Lamprini; Robberstad, Bjarne; Steens, Anneke; Forland, Frode; Winje, Brita A. & Vestrheim, Didrik Frimann [Show all 11 contributors for this article] (2024). Social contact patterns during the early COVID-19 pandemic in Norway: insights from a panel study, April to September 2020. BMC Public Health. 24(1). doi: 10.1186/s12889-024-18853-8. Full text in Research Archive Show summary

  Background During the COVID-19 pandemic, many countries adopted social distance measures and lockdowns of varying strictness. Social contact patterns are essential in driving the spread of respiratory infections, and country-specific measurements are needed. This study aimed to gain insights into changes in social contacts and behaviour during the early pandemic phase in Norway. Methods We conducted an online panel study among a nationally representative sample of Norwegian adults by age and gender. The panel study included six data collections waves between April and September 2020, and 2017 survey data from a random sample of the Norwegian population (including children < 18 years old) were used as baseline. The market research company Ipsos was responsible for carrying out the 2020 surveys. We calculated mean daily contacts, and estimated age-stratified contact matrices during the study period employing imputation of child-to-child contacts. We used the next-generation method to assess the relative reduction of R0 and compared the results to reproduction numbers estimated for Norway during the 2020 study period. Results Over the six waves in 2020, 5 938 observations/responses were registered from 1 718 individuals who reported data on 22 074 contacts. The mean daily number of contacts among adults varied between 3.2 (95%CI 3.0-3.4) to 3.9 (95%CI 3.6–4.2) across the data collection waves, representing a 67–73% decline compared to pre-pandemic levels (baseline). Fewer contacts in the community setting largely drove the reduction; the drop was most prominent among younger adults. Despite gradual easing of social distance measures during the survey period, the estimated population contact matrices remained relatively stable and displayed more inter-age group mixing than at baseline. Contacts within households and the community outside schools and workplaces contributed most to social encounters. Using the next-generation method R0 was found to be roughly 25% of pre-pandemic levels during the study period, suggesting controlled transmission. Conclusion Social contacts declined significantly in the months following the March 2020 lockdown, aligning with implementation of stringent social distancing measures. These findings contribute valuable empirical information into the social behaviour in Norway during the early pandemic, which can be used to enhance policy-relevant models for addressing future crises when mitigation measures might be implemented.

• Blasio, Fabio V De & Blasio, Birgitte Freiesleben De (2024). Rapid clade divergence and phyletic gradualism in an interacting particle model of sympatric speciation. Biosystems (Amsterdam. Print). ISSN 0303-2647. 239. doi: 10.1016/j.biosystems.2024.105198. Full text in Research Archive Show summary

  The coexistence of cladogenesis, i.e., the branching of lineages along an evolutionary tree as observed in the fossil record, and anagenesis, which is the progressive evolution within populations, lacks a clear explanation. In this study, we examine a simple model that simulates the evolutionary changes occurring within populations inhabiting the same environment in sympatry, and driven by ecological competition. Our model characterizes populations through a set of evolving morphological traits represented by mathematical points within a two-dimensional morphospace. Such points may reproduce or die due to overcrowding, implying competition in morphospace as suggested by the ecological phenomenon of character displacement. By focusing on the morphospace rather than physical space, the model effectively captures the simultaneous evolution of coexisting populations. Central to the model is the delicate balance between the range of competition and the range of reproduction within the morphospace. Interesting patterns emerge when the ratio between the competition to reproducetion ranges, referred to as CR ratio, changes from values slightly smaller to significantly larger than unity. When competition acts over short distances relative to the reproduction range (low CR), the phylogenetic tree takes on a nearly uniform appearance, gradually transforming into a more bush-like structure for slightly higher CR values. With further increases in CR, evolutionary lineages become more discernible, and the morphogenetic pattern shifts from a bush-like shape to a more tree-like arrangement and few branches for very large CRs. At specific time sections, the synthetic phylogenetic tree appears as an assembly of clusters of individuals within the morphospace. These clusters, interpretable as simulated models of species, exhibit distinct separation within the morphospace and are subject to dynamic inter-cluster repulsion. Notably, clusters tend to be resistant to change. They maintain relatively constant abundances while gradually shifting their positions within the morphospace—a phase that aligns with the concept of phyletic gradualism. However, this predictable pattern is occasionally upset by the abrupt divisions into multiple groups, interpreted as cladogenesis events. The intricacies of the splitting process are explored, revealing that in scenarios with large CR values, the splitting can emerge much more rapidly than phyletic changes. This accelerated process of splitting is initiated by one or few individuals at the fringes of a cluster, where competition is minimal. The newly generated cluster then undergoes deformation, swiftly followed by divergence and splitting (seen as branching in the synthetic phylogenetic tree), as if an inherent "repulsion" triggered the division between species. The simple rules implied in the interacting-particle model may provide insight into the coexistence of gradualism and cladogenesis along lineages, illustrating the capacity for rapid shifts during cladogenesis and the more gradual process of anagenesis.

• Kamineni, Meghana; Engø-Monsen, Kenth; Midtbø, Jørgen E.; Forland, Frode; Blasio, Birgitte Freiesleben De & Frigessi, Arnoldo [Show all 7 contributors for this article] (2023). Effects of non-compulsory and mandatory COVID-19 interventions on travel distance and time away from home, Norway, 2021. Eurosurveillance. ISSN 1025-496X. 28(17). doi: 10.2807/1560-7917.ES.2023.28.17.2200382. Full text in Research Archive Show summary

  Background Given the societal, economic and health costs of COVID-19 non-pharmaceutical interventions (NPI), it is important to assess their effects. Human mobility serves as a surrogate measure for human contacts and compliance with NPI. In Nordic countries, NPI have mostly been advised and sometimes made mandatory. It is unclear if making NPI mandatory further reduced mobility. Aim We investigated the effect of non-compulsory and follow-up mandatory measures in major cities and rural regions on human mobility in Norway. We identified NPI categories that most affected mobility. Methods We used mobile phone mobility data from the largest Norwegian operator. We analysed non-compulsory and mandatory measures with before–after and synthetic difference-in-differences approaches. By regression, we investigated the impact of different NPI on mobility. Results Nationally and in less populated regions, time travelled, but not distance, decreased after follow-up mandatory measures. In urban areas, however, distance decreased after follow-up mandates, and the reduction exceeded the decrease after initial non-compulsory measures. Stricter metre rules, gyms reopening, and restaurants and shops reopening were significantly associated with changes in mobility. Conclusion Overall, distance travelled from home decreased after non-compulsory measures, and in urban areas, distance further decreased after follow-up mandates. Time travelled reduced more after mandates than after non-compulsory measures for all regions and interventions. Stricter distancing and reopening of gyms, restaurants and shops were associated with changes in mobility.

• Storvik, Geir Olve; Palomares, Alfonso Diz-Lois; Engebretsen, Solveig; Rø, Gunnar Øyvind Isaksson; Engø-Monsen, Kenth & Kristoffersen, Anja Bråthen [Show all 8 contributors for this article] (2023). A sequential Monte Carlo approach to estimate a time-varying reproduction number in infectious disease models: the Covid-19 case. Journal of the Royal Statistical Society, Series A (Statistics in Society). ISSN 0964-1998. 186(4), p. 616–632. doi: 10.1093/jrsssa/qnad043. Full text in Research Archive Show summary

  Abstract The Covid-19 pandemic has required most countries to implement complex sequences of non-pharmaceutical interventions, with the aim of controlling the transmission of the virus in the population. To be able to take rapid decisions, a detailed understanding of the current situation is necessary. Estimates of time-varying, instantaneous reproduction numbers represent a way to quantify the viral transmission in real time. They are often defined through a mathematical compartmental model of the epidemic, like a stochastic SEIR model, whose parameters must be estimated from multiple time series of epidemiological data. Because of very high dimensional parameter spaces (partly due to the stochasticity in the spread models) and incomplete and delayed data, inference is very challenging. We propose a state-space formalization of the model and a sequential Monte Carlo approach which allow to estimate a daily-varying reproduction number for the Covid-19 epidemic in Norway with sufficient precision, on the basis of daily hospitalization and positive test incidences. The method was in regular use in Norway during the pandemics and appears to be a powerful instrument for epidemic monitoring and management.

• Osnes, Magnus Nygård; Brynildsrud, Ola Brønstad; Alfsnes, Kristian; Dorp, Lucy van; McKeand, Samantha A & Ross, Jonathan [Show all 17 contributors for this article] (2023). The inter-continental population dynamics of Neisseria gonorrhoeae. eLIFE. doi: 10.7554/eLife.90062.1. Full text in Research Archive
• Engebretsen, Solveig; Palomares, Alfonso Diz-Lois; Rø, Gunnar Øyvind Isaksson; Kristoffersen, Anja Bråthen; Lindstrøm, Jonas Christoffer & Engø-Monsen, Kenth [Show all 15 contributors for this article] (2023). A real-time regional model for COVID-19: Probabilistic situational awareness and forecasting. PLoS Computational Biology. ISSN 1553-734X. 19(1). doi: 10.1371/journal.pcbi.1010860. Full text in Research Archive Show summary

  The COVID-19 pandemic is challenging nations with devastating health and economic consequences. The spread of the disease has revealed major geographical heterogeneity because of regionally varying individual behaviour and mobility patterns, unequal meteorological conditions, diverse viral variants, and locally implemented non-pharmaceutical interventions and vaccination roll-out. To support national and regional authorities in surveilling and controlling the pandemic in real-time as it unfolds, we here develop a new regional mathematical and statistical model. The model, which has been in use in Norway during the first two years of the pandemic, is informed by real-time mobility estimates from mobile phone data and laboratory-confirmed case and hospitalisation incidence. To estimate regional and time-varying transmissibility, case detection probabilities, and missed imported cases, we developed a novel sequential Approximate Bayesian Computation method allowing inference in useful time, despite the high parametric dimension. We test our approach on Norway and find that three-week-ahead predictions are precise and well-calibrated, enabling policy-relevant situational awareness at a local scale. By comparing the reproduction numbers before and after lockdowns, we identify spatially heterogeneous patterns in their effect on the transmissibility, with a stronger effect in the most populated regions compared to the national reduction estimated to be 85% (95% CI 78%-89%). Our approach is the first regional changepoint stochastic metapopulation model capable of real time spatially refined surveillance and forecasting during emergencies.

• Debes, Sara Molvig; Haug, Jon Birger; Blasio, Birgitte Freiesleben de; Lindstrøm, Jonas Christoffer; Jonassen, Christine Genevieve Monceyron & Dudman, Susanne Gjeruldsen (2023). Antibiotic Consumption in a Cohort of Hospitalized Adults with Viral Respiratory Tract Infection. Antibiotics. 12(4). doi: 10.3390/antibiotics12040788. Full text in Research Archive Show summary

  Development of antibiotic resistance, a threat to global health, is driven by inappropriate antibiotic usage. Respiratory tract infections (RTIs) are frequently treated empirically with antibiotics, despite the fact that a majority of the infections are caused by viruses. The purpose of this study was to determine the prevalence of antibiotic treatment in hospitalized adults with viral RTIs, and to investigate factors influencing the antibiotic decision-making. We conducted a retrospective observational study of patients ≥ 18 years, hospitalized in 2015–2018 with viral RTIs. Microbiological data were taken from the laboratory information system and information on antibiotic treatment drawn from the hospital records. To investigate decisions for prescribing antibiotic treatment, we evaluated relevant factors such as laboratory and radiological results, in addition to clinical signs. In 951 cases without secondary bacterial RTIs (median age 73 years, 53% female), 720 (76%) were prescribed antibiotic treatment, most frequently beta-lactamase-sensitive penicillins, but cephalosporins were prescribed as first-line in 16% of the cases. The median length of treatment (LOT) in the patients treated with antibiotics was seven days. Patients treated with antibiotics had an average of two days longer hospital stay compared to patients with no such treatment, but no difference in mortality was found. Our study revealed that there is still a role for antimicrobial stewardship to further improve antibiotic use in patients admitted for viral RTIs in a country with relatively low antibiotic consumption.

• Aarø, Leif Edvard; Veneti, Lamprini; Vedaa, Øystein; Smith, Otto Robert Frans; Blasio, Birgitte Freiesleben De & Robberstad, Bjarne (2022). Visiting crowded places during the COVID-19 pandemic. A panel study among adult Norwegians. Frontiers in Public Health. 10. doi: 10.3389/fpubh.2022.1076090. Full text in Research Archive Show summary

  Non-pharmaceutical interventions, including promotion of social distancing, have been applied extensively in managing the COVID-19 pandemic. Understanding cognitive and psychological factors regulating precautionary behavior is important for future management. The present study examines the importance of selected factors as predictors of having visited or intended to visit crowded places. Six online questionnaire-based waves of data collection were conducted in April–October 2020 in a Norwegian panel (≥18 years). Sample size at Wave 1 was 1,400. In the present study, “Visited or intended to visit crowded places” for different types of locations were the dependent variables. Predictors included the following categories of items: Perceived response effectiveness, Self-efficacy, Vulnerability, Facilitating factors and Barriers. Data were analyzed with frequency and percentage distributions, descriptives, correlations, principal components analysis, negative binomial-, binary logistic-, and multiple linear regression, and cross-lagged panel models. Analyses of dimensionality revealed that a distinction had to be made between Grocery stores, a location visited by most, and locations visited by few (e.g., “Pub,” “Restaurants,” “Sports event”). We merged the latter set of variables into a countscore denoted as “Crowded places.” On the predictor side, 25 items were reduced to eight meanscores. Analyses of data from Wave 1 revealed a rather strong prediction of “Crowded places” and weaker associations with “Supermarket or other store for food.” Across waves, in multiple negative binomial regression models, three meanscore predictors turned out to be consistently associated with “Crowded places.” These include “Response effectiveness of individual action,” “Self-efficacy with regard to avoiding people,” and “Barriers.” In a prospective cross-lagged model, a combined Response effectiveness and Self-efficacy score (Cognition) predicted behavior (“Visited or intended to visit crowded places”) prospectively and vice versa. The results of this study suggest some potential to reduce people's visits to crowded locations during the pandemic through health education and behavior change approaches that focus on strengthening individuals' perceived response effectiveness and self-efficacy.

• Duchene, Sebastian; Featherstone, Leo; Blasio, Birgitte Freiesleben de; Holmes, Edward C.; Bohlin, Jon & Pettersson, John H.-O. (2022). Assessment of Coronavirus Disease 2019 Intervention Strategies in the Nordic Countries Using Genomic Epidemiology. Open Forum Infectious Diseases. 9(3), p. 1–6. doi: 10.1093/ofid/ofab665. Full text in Research Archive Show summary

  We explored how the duration, size, and number of virus transmission clusters, defined as country-specific monophyletic groups in a severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) phylogenetic tree, differed among the Nordic countries of Norway, Sweden, Denmark, Finland, and Iceland. Our results suggest that although geographical connectivity, population density, and openness influence the spread and the size of SARS-CoV-2 transmission clusters, the different country-specific intervention strategies had the largest impact. We also found a significant positive association between the size and duration of transmission clusters in the Nordic countries, suggesting that the rapid deployment of contact tracing is a key response measure in reducing virus transmission.

• Debes, Sara Molvig; Haug, Jon Birger; Blasio, Birgitte Freiesleben De; Lindstrøm, Jonas Christoffer; Jonassen, Christine M & Dudman, Susanne Gjeruldsen (2022). Clinical Outcome of Viral Respiratory Tract Infections in Hospitalized Adults in Norway: High Degree of Inflammation and Need of Emergency Care for Cases With Respiratory Syncytial Virus. Frontiers in Medicine. 9. doi: 10.3389/fmed.2022.866494. Full text in Research Archive Show summary

  Background: The clinical features and outcomes of viral respiratory tract infections (RTIs) in adults have not been thoroughly studied, especially the respiratory syncytial virus (RSV) disease burden. It has become apparent that outbreaks of RSV in the elderly are associated with increased hospitalization rates. However, little data exists on the severity of such viral RTIs in adults, particularly the need for hospitalization, respiratory support and intensive care. Methods: We conducted a retrospective observational single-center study at Østfold Hospital Trust, Norway, during three winter seasons 2015–2018. Patients ≥18 years with either influenza A, influenza B, RSV A/B, human metapneumovirus, parainfluenza virus 1–4 or adenovirus detected in respiratory specimens were included, if they were hospitalized 14 days prior or following the detection date, with signs of RTI. Hospital records on treatment and outcome were investigated, as well as mortality of all causes up to 30 days from discharge. Results: Of the 1222 infection events that were included, influenza A was the most frequent virus detected (39%), while 179 infection events (14.6%) were due to RSV. Influenza B counted for 24% of the infection events, human metapneumovirus 13%, parainfluenza virus 9% and adenovirus 1%. Patients admitted with RSV more often suffered from COPD and congestive heart failure than patients with influenza A. In addition, RSV patients were overrepresented in the urgent response NEWS score (National Early Warning Score) category ≥5. RSV patients also showed signs of more severe inflammation, with WBC ≥11.1 × 109/L and CRP >100 mg/L, and they were more often treated with antibiotic agents during their hospital stay. However, we found no differences in the need for ICU admission or mortality. Conclusion: Patients with RSV had more often high values for markers of inflammation and elevated NEWS score when compared to patients hospitalized with other common respiratory viruses. Taken into account that they suffered more frequently from comorbidities like COPD, these patients needed hospitalization more urgently. These findings highlight the need for further investigations on RSV disease in adults and the elderly.

• Seppälä, Elina Marjukka; Kristoffersen, Anja Bråthen; Bøås, Håkon; Vestrheim, Didrik Frimann; Greve-Isdahl, Margrethe & Blasio, Birgitte Freiesleben De [Show all 7 contributors for this article] (2022). Pertussis epidemiology including direct and indirect effects of the childhood pertussis booster vaccinations, Norway, 1998–2019. Vaccine. ISSN 0264-410X. 40(23), p. 3142–3149. doi: 10.1016/j.vaccine.2022.04.038. Full text in Research Archive
• Engebretsen, Solveig; Rø, Gunnar Øyvind Isaksson & Blasio, Birgitte Freiesleben De (2022). A compelling demonstration of why traditional statistical regression models cannot be used to identify risk factors from case data on infectious diseases: a simulation study. BMC Medical Research Methodology. 22. doi: 10.1186/s12874-022-01565-1. Full text in Research Archive Show summary

  Background: Regression models are often used to explain the relative risk of infectious diseases among groups. For example, overrepresentation of immigrants among COVID-19 cases has been found in multiple countries. Several studies apply regression models to investigate whether different risk factors can explain this overrepresentation among immigrants without considering dependence between the cases. Methods: We study the appropriateness of traditional statistical regression methods for identifying risk factors for infectious diseases, by a simulation study. We model infectious disease spread by a simple, population-structured version of an SIR (susceptible-infected-recovered)-model, which is one of the most famous and well-established models for infectious disease spread. The population is thus divided into different sub-groups. We vary the contact structure between the sub-groups of the population. We analyse the relation between individual-level risk of infection and group-level relative risk. We analyse whether Poisson regression estimators can capture the true, underlying parameters of transmission. We assess both the quantitative and qualitative accuracy of the estimated regression coefficients. Results: We illustrate that there is no clear relationship between differences in individual characteristics and group-level overrepresentation —small differences on the individual level can result in arbitrarily high overrepresentation. We demonstrate that individual risk of infection cannot be properly defined without simultaneous specification of the infection level of the population. We argue that the estimated regression coefficients are not interpretable and show that it is not possible to adjust for other variables by standard regression methods. Finally, we illustrate that regression models can result in the significance of variables unrelated to infection risk in the constructed simulation example (e.g. ethnicity), particularly when a large proportion of contacts is within the same group. Conclusions: Traditional regression models which are valid for modelling risk between groups for non-communicable diseases are not valid for infectious diseases. By applying such methods to identify risk factors of infectious diseases, one risks ending up with wrong conclusions. Output from such analyses should therefore be treated with great caution.

• Jalali, Neda; Brustad, Hilde Kjelgaard; Frigessi, Arnoldo; Macdonald, Emily Ann; Meijerink, Hinta & Feruglio, Siri Laura [Show all 10 contributors for this article] (2022). Increased household transmission and immune escape of the SARS-CoV-2 Omicron compared to Delta variants. Nature Communications. 13(1), p. 1–5. doi: 10.1038/s41467-022-33233-9. Full text in Research Archive Show summary

  Understanding the epidemic growth of the novel SARS-CoV-2 Omicron variant is critical for public health. We compared the ten-day secondary attack rate (SAR) of the Omicron and Delta variants in households using Norwegian contact tracing data, December 2021 - January 2022. Omicron SAR was higher than Delta, with a relative risk (RR) of 1.41 (95% CI 1.27-1.56). We observed increased susceptibility to Omicron infection in household contacts compared to Delta, independent of contacts’ vaccination status. Among three-dose vaccinated contacts, the mean SAR was lower for both variants. We found increased Omicron transmissibility from primary cases to contacts in all vaccination groups, except 1-dose vaccinated, compared to Delta. Omicron SAR of three-dose vaccinated primary cases was high, 46% vs 11 % for Delta. In conclusion, three-dose vaccinated primary cases with Omicron infection can efficiently spread in households, while three-dose vaccinated contacts have a lower risk of being infected by Delta and Omicron.

• Lindstrøm, Jonas Christoffer; Engebretsen, Solveig; Kristoffersen, Anja Bråthen; Rø, Gunnar Øyvind Isaksson; Palomares, Alfonso Diz-Lois & Engø-Monsen, Kenth [Show all 14 contributors for this article] (2021). Increased transmissibility of the alpha SARS-CoV-2 variant: evidence from contact tracing data in Oslo, January to February 2021. Infectious Diseases. ISSN 2374-4235. 54(1), p. 72–77. doi: 10.1080/23744235.2021.1977382. Full text in Research Archive Show summary

  Background: Information about the contagiousness of new SARS-CoV-2 variants, including the alpha lineage, and how they spread in various locations is essential. Country-specific estimates are needed because local interventions influence transmissibility. Methods: We analysed contact tracing data from Oslo municipality, reported from January through February 2021, when the alpha lineage became predominant in Norway and estimated the relative transmissibility of the alpha lineage with the use of Poisson regression. Results: Within households, we found an increase in the secondary attack rate by 60% (95% CI 20–114%) among cases infected with the alpha lineage compared to other variants; including all close contacts, the relative increase in the secondary attack rate was 24% (95% CI −6%−43%). There was a significantly higher risk of infecting household members in index cases aged 40–59 years who were infected with the alpha lineage; we found no association between transmission and household size. Overall, including all close contacts, we found that the reproduction number among cases with the alpha lineage was increased by 24% (95% CI 0%−52%), corresponding to an absolute increase of 0.19, compared to the group of index cases infected with other variants. Conclusion: Our study suggests that households are the primary locations for rapid transmission of the new lineage alpha.

• Duchêne, Sebastián; Featherstone, Leo; Blasio, Birgitte Freiesleben de; Holmes, Edward C; Bohlin, Jon & Pettersson, John H.-O. (2021). The impact of public health interventions in the Nordic countries during the first year of SARS-CoV-2 transmission and evolution. Eurosurveillance. ISSN 1025-496X. 26(44). doi: 10.2807/1560-7917.ES.2021.26.44.2001996. Full text in Research Archive Show summary

  Background Many countries have attempted to mitigate and control COVID-19 through non-pharmaceutical interventions, particularly with the aim of reducing population movement and contact. However, it remains unclear how the different control strategies impacted the local phylodynamics of the causative SARS-CoV-2 virus. Aim We aimed to assess the duration of chains of virus transmission within individual countries and the extent to which countries exported viruses to their geographical neighbours. Methods We analysed complete SARS-CoV-2 genomes to infer the relative frequencies of virus importation and exportation, as well as virus transmission dynamics, in countries of northern Europe. We examined virus evolution and phylodynamics in Denmark, Finland, Iceland, Norway and Sweden during the first year of the COVID-19 pandemic. Results The Nordic countries differed markedly in the invasiveness of control strategies, which we found reflected in transmission chain dynamics. For example, Sweden, which compared with the other Nordic countries relied more on recommendation-based rather than legislation-based mitigation interventions, had transmission chains that were more numerous and tended to have more cases. This trend increased over the first 8 months of 2020. Together with Denmark, Sweden was a net exporter of SARS-CoV-2. Norway and Finland implemented legislation-based interventions; their transmission chain dynamics were in stark contrast to their neighbouring country Sweden. Conclusion Sweden constituted an epidemiological and evolutionary refugium that enabled the virus to maintain active transmission and spread to other geographical locations. Our analysis reveals the utility of genomic surveillance where monitoring of active transmission chains is a key metric.

• Debes, Sara Molvig; Haug, Jon Birger; Blasio, Birgitte Freiesleben De; Jonassen, Christine M & Dudman, Susanne Gjeruldsen (2021). Etiology of viral respiratory tract infections in hospitalized adults, and evidence of the high frequency of prehospitalization antibiotic treatment in Norway. Health Science Reports. 4(4). doi: 10.1002/hsr2.403. Full text in Research Archive
• Hauge, Siri H.; Blasio, Birgitte Freiesleben de; Håberg, Siri Eldevik & Oakley, Laura L. (2021). Influenza hospitalizations during childhood in children born preterm. Influenza and Other Respiratory Viruses. ISSN 1750-2640. doi: 10.1111/irv.12908. Full text in Research Archive Show summary

  Objective The objective is to determine if children born preterm were at increased risk of influenza hospitalization up to age five. Methods National registry data on all children born in Norway between 2008 and 2011 were used in Cox regression models to estimate adjusted hazard ratios (aHRs) for influenza hospitalizations up to age five in children born preterm (<37 pregnancy weeks). HRs were also estimated separately for very preterm (<32 weeks), early term (37–38 weeks), and post-term (≥42 weeks) children. Results Among 238,628 children born in Norway from January 2008 to December 2011, 15,086 (6.3%) were born preterm. There were 754 (0.3%) children hospitalized with influenza before age five. The rate of hospitalizations in children born preterm was 13.8 per 10,000 person-years (95% confidence interval [CI] [11.3, 16.7]), and 5.9 per 10,000 person-years (95% CI [5.5, 6.4]) in children born at term (≥37 weeks). Children born preterm had a higher risk of influenza hospitalization before age 5: aHR 2.33 (95% CI [1.85, 2.93]). The risk increased with decreasing gestational age and was highest among those born extremely/very preterm; aHR 4.07 (95% CI [2.63, 6.31]). Compared with children born at 40–41 weeks, children born early term also had an elevated risk of influenza hospitalization; aHR (37 weeks) 1.89 (95% CI [1.43, 2.50]), aHR (38 weeks) 1.43 (95% CI [1.15, 1.78]). Conclusion Children born preterm had a higher risk of influenza hospitalizations before age five. An elevated risk was also present among children born at an early term. Children born preterm could benefit from influenza vaccinations.

• Engebretsen, Solveig; Engø-Monsen, Kenth; Aleem, Mohammad Abdul; Gurley, Emily Suzanne; Rattalma, Arnoldo Frigessi Di & Blasio, Birgitte Freiesleben de (2020). Time-aggregated mobile phone mobility data are sufficient for modelling influenza spread: the case of Bangladesh. Journal of the Royal Society Interface. ISSN 1742-5689. 17(167). doi: 10.1098/rsif.2019.0809. Full text in Research Archive Show summary

  Human mobility plays a major role in the spatial dissemination of infectious diseases. We develop a spatio-temporal stochastic model for influenza-like disease spread based on estimates of human mobility. The model is informed by mobile phone mobility data collected in Bangladesh. We compare predictions of models informed by daily mobility data (reference) with that of models informed by time-averaged mobility data, and mobility model approximations. We find that the gravity model overestimates the spatial synchrony, while the radiation model underestimates the spatial synchrony. Using time-averaged mobility resulted in spatial spreading patterns comparable to the daily mobility model. We fit the model to 2014–2017 influenza data from sentinel hospitals in Bangladesh, using a sequential version of approximate Bayesian computation. We find a good agreement between our estimated model and the case data. We estimate transmissibility and regional spread of influenza in Bangladesh, which are useful for policy planning. Time-averaged mobility appears to be a good proxy for human mobility when modelling infectious diseases. This motivates a more general use of the time-averaged mobility, with important implications for future studies and outbreak control. Moreover, time-averaged mobility is subject to less privacy concerns than daily mobility, containing less temporal information on individual movements

• Hauge, Siri Helene; Bakken, Inger Johanne; Blasio, Birgitte Freiesleben De & Håberg, Siri Eldevik (2020). Risk conditions in children hospitalized with influenza in Norway, 2017–2019. BMC Infectious Diseases. 20(1). doi: 10.1186/s12879-020-05486-6. Full text in Research Archive Show summary

  Background Norwegian children are more frequently hospitalized with influenza than adults. Little is known about the characteristics of these children. Our aim was to investigate the presence of pre-existing risk conditions and to determine the duration of influenza hospitalizations in children during two influenza seasons. Methods The Norwegian Patient Registry holds data on all hospitalized patients in Norway. We included all patients younger than 18 years hospitalized with a diagnosis of influenza during the influenza seasons 2017–18 and 2018–19. Pre-existing risk conditions for influenza were identified by ICD-10 diagnoses in the Norwegian Patient Registry. In addition, information on asthma diagnoses were also retrieved from the Norwegian Registry for Primary Health Care. To estimate the prevalence of risk conditions in the child population, we obtained diagnoses on all Norwegian children in a two-year period prior to each influenza season. We calculated age-specific rates for hospitalization and risk for being hospitalized with influenza in children with risk conditions. Results In total, 1013 children were hospitalized with influenza during the two influenza seasons. Children younger than 6 months had the highest rate of hospitalization, accounting for 13.5% of all admissions (137 children). Hospitalization rates decreased with increasing age. Among children hospitalized with influenza, 25% had one or more pre-existing risk conditions for severe influenza, compared to 5% in the general population under 18 years. Having one or more risk conditions significantly increased the risk of hospitalization, (Odds Ratio (OR) 6.1, 95% confidence interval (CI) 5.0–7.4 in the 2017–18 season, and OR 6.8, 95% CI 5.4–8.4 in the 2018–19 season). Immunocompromised children and children with epilepsy had the highest risk of hospitalization with influenza, followed by children with heart disease and lung disease. The average length of stay in hospital were 4.6 days, and this did not differ with age. Conclusion Children with pre-existing risk conditions for influenza had a higher risk of hospitalization for influenza. However, most children (75%) admitted to hospital with influenza in Norway during 2017–2019 did not have pre-existing risk conditions. Influenza vaccination should be promoted in particular for children with risk conditions and pregnant women to protect new-borns.

• Bohlin, Jon; Rose, Brittany; Brynildsrud, Ola Brønstad & Blasio, Birgitte Freiesleben de (2020). A simple stochastic model describing genomic evolution over time of GC content in microbial symbionts. Journal of Theoretical Biology. ISSN 0022-5193. 503, p. 1–8. doi: 10.1016/j.jtbi.2020.110389. Full text in Research Archive Show summary

  An organism’s genomic base composition is usually summarized by its AT or GC content due to Chargaff’s parity laws. Variation in prokaryotic GC content can be substantial between taxa but is generally small within microbial genomes. This variation has been found to correlate with both phylogeny and environmental factors. Since novel single-nucleotide polymorphisms (SNPs) within genomes are at least partially linked to the environment through natural selection, SNP GC content can be considered a compound measure of an organism’s environmental influences, lifestyle, phylogeny as well as other more or less random processes. While there are several models describing genomic GC content few, if any, consider AT/GC mutation rates subjected to random perturbations. We present a mathematical model that describes how GC content in microbial genomes evolves over time as a function of the AT → GC and GC → AT mutation rates with Gaussian white noise disturbances. The model, which is suited specifically to non-recombining vertically transmitted prokaryotic symbionts, suggests that small differences in the AT/GC mutation rates can lead to profound differences in outcome due to the ensuing stochastic process. In other words, the model indicates that time to extinction could be a consequence of the mutation rate trajectory on which the symbiont embarked early on in its evolutionary history.

• Osnes, Magnus Nygård; Didelot, Xavier; Korne-Elenbaas, Jolinda de; Alfsnes, Kristian; Brynildsrud, Ola Brønstad & Syversen, Gaute [Show all 10 contributors for this article] (2020). Sudden emergence of a Neisseria gonorrhoeae clade with reduced susceptibility to extended-spectrum cephalosporins, Norway. Microbial Genomics. 6(12). doi: 10.1099/mgen.0.000480. Full text in Research Archive Show summary

  Neisseria gonorrhoeae multilocus sequence type (ST)-7827 emerged in a dramatic fashion in Norway in the period 2016–2018. Here, we aim to shed light on the provenance and expansion of this ST. ST-7827 was found to be polyphyletic, but the majority of members belonged to a monophyletic clade we termed PopPUNK cluster 7827 (PC-7827). In Norway, both PC-7827 and ST-7827 isolates were almost exclusively isolated from men. Phylogeographical analyses demonstrated an Asian origin of the genogroup, with multiple inferred exports to Europe and the USA. The genogroup was uniformly resistant to fluoroquinolones, and associated with reduced susceptibility to both azithromycin and the extended-spectrum cephalosporins (ESCs) cefixime and ceftriaxone. From a genetic background including the penA allele 13.001, associated with reduced ESC susceptibility, we identified repeated events of acquisition of porB alleles associated with further reduction in ceftriaxone susceptibility. Transmission of the strain was significantly reduced in Norway in 2019, but our results indicate the existence of a recently established global reservoir. The worrisome drug-resistance profile and rapid emergence of PC-7827 calls for close monitoring of the situation.

• Duchêne, Sebastián; Featherstone, Leo; Blasio, Birgitte Freiesleben de; Holmes, Edward C.; Bohlin, Jon & Pettersson, John H.-O. (2020). The impact of early public health interventions on SARS-CoV-2 transmission and evolution. medRxiv. doi: 10.1101/2020.11.18.20233767. Full text in Research Archive
• Steens, Anneke; Blasio, Birgitte Freiesleben De; Veneti, Lamprini; Gimma, Amy; Edmunds, W. John & Zandvoort, Kevin Van [Show all 9 contributors for this article] (2020). Poor self-reported adherence to COVID-19-related quarantine/isolation requests, Norway, April to July 2020. Eurosurveillance. ISSN 1025-496X. 25(37), p. 1–6. doi: 10.2807/1560-7917.ES.2020.25.37.2001607. Full text in Research Archive Show summary

  To limit SARS-CoV-2 spread, quarantine and isolation are obligatory in several situations in Norway. We found low self-reported adherence to requested measures among 1,704 individuals (42%; 95% confidence interval: 37–48). Adherence was lower in May–June–July (33–38%) compared with April (66%), and higher among those experiencing COVID-19-compatible symptoms (71%) compared with those without (28%). These findings suggest that consideration is required of strategies to improve people’s adherence to quarantine and isolation.

• Ruscio, Francesco Di; Guzzetta, Giorgio; Bjørnholt, Jørgen; Leegaard, Truls Michael; Moen, Aina Elisabeth Fossum & Merler, Stefano [Show all 7 contributors for this article] (2019). Quantifying the transmission dynamics of MRSA in the community and healthcare settings in a low-prevalence country. Proceedings of the National Academy of Sciences of the United States of America (PNAS). ISSN 0027-8424. 116(29), p. 14599–14605. doi: 10.1073/pnas.1900959116. Full text in Research Archive Show summary

  Methicillin-resistant Staphylococcus aureus (MRSA) is a primarily nosocomial pathogen that, in recent years, has increasingly spread to the general population. The rising prevalence of MRSA in the community implies more frequent introductions in healthcare settings that could jeopardize the effectiveness of infection-control procedures. To investigate the epidemiological dynamics of MRSA in a low-prevalence country, we developed an individual-based model (IBM) reproducing the population’s sociodemography, explicitly representing households, hospitals, and nursing homes. The model was calibrated to surveillance data from the Norwegian national registry (2008–2015) and to published household prevalence data. We estimated an effective reproductive number of 0.68 (95% CI 0.47–0.90), suggesting that the observed rise in MRSA infections is not due to an ongoing epidemic but driven by more frequent acquisitions abroad. As a result of MRSA importations, an almost twofold increase in the prevalence of carriage was estimated over the study period, in 2015 reaching a value of 0.37% (0.25–0.54%) in the community and 1.11% (0.79–1.59%) in hospitalized patients. Household transmission accounted for half of new MRSA acquisitions, indicating this setting as a potential target for preventive strategies. However, nosocomial acquisition was still the primary source of symptomatic disease, which reinforces the importance of hospital-based transmission control. Although our results indicate little reason for concern about MRSA transmission in low-prevalence settings in the immediate future, the increases in importation and global circulation highlight the need for coordinated initiatives to reduce the spread of antibiotic resistance worldwide.

• Edwards, Christina Hansen; Tomba, Gianpaolo Scalia; Kristiansen, Ivar Sønbø; White, Richard Aubrey & Blasio, Birgitte Freiesleben De (2019). Evaluating costs and health consequences of sick leave strategies against pandemic and seasonal influenza in Norway using a dynamic model. BMJ Open. 9. doi: 10.1136/bmjopen-2018-027832. Full text in Research Archive Show summary

  Objectives: To quantify population-level health and economic consequences of sick leave among workers with influenza symptoms. Interventions: Compared with current sick leave practice (baseline), we evaluated the health and cost consequences of: (1) increasing the proportion of workers on sick leave from 65% (baseline) to 80% or 90%; (2) shortening the maximum duration from symptom onset to sick leave from 4 days (baseline) to 2 days, 1.5 days, 1 day and 0.5 days; and (3) combinations of 1 and 2. Methods: A dynamic compartmental influenza model was developed using Norwegian population data and survey data on employee sick leave practices. The sick leave interventions were simulated under 12 different seasonal epidemic and 36 different pandemic influenza scenarios. These scenarios varied in terms of transmissibility, the proportion of symptomatic cases and illness severity (risk of primary care consultations, hospitalisations and deaths). Using probabilistic sensitivity analyses, a net health benefit approach was adopted to assess the cost-effectiveness of the interventions from a societal perspective. Results: Compared with current sick leave practice, sick leave interventions were cost-effective for 31 (65%) of the pandemic scenarios, and 11 (92%) of the seasonal scenarios. Economic benefits from sick leave interventions were greatest for scenarios with low transmissibility, high symptomatic proportions and high illness severity. Overall, the health and economic benefits were greatest for the intervention involving 90% of sick workers taking sick leave within one-half day of symptoms. Depending on the influenza scenario, this intervention resulted in a 44.4%–99.7% reduction in the attack rate. Interventions involving sick leave onset beginning 2 days or later, after the onset of symptoms, resulted in economic losses. Conclusions: Prompt sick leave onset and a high proportion of sick leave among workers with influenza symptoms may be cost-effective, particularly during influenza epidemics and pandemics with low transmissibility or high morbidity.

• Bohlin, Jon; Rose, Brittany; Brynildsrud, Ola Brønstad & Blasio, Birgitte Freiesleben De (2019). A simple stochastic model to describe the evolutionover time of core genome SNP GC content inprokaryotes. arXiv.org. ISSN 2331-8422. doi: 10.1016/j.jtbi.2020.110389. Full text in Research Archive
• Hauge, Siri Helene; Bakken, Inger Johanne Landsjøåsen; Blasio, Birgitte Freiesleben De & Håberg, Siri Eldevik (2019). Burden of medically attended influenza in Norway 2008‐2017. Influenza and Other Respiratory Viruses. ISSN 1750-2640. 13(3), p. 240–247. doi: 10.1111/irv.12627. Full text in Research Archive Show summary

  Background The burden of influenza in Norway remains uncertain, and data on seasonal variations and differences by age groups are needed. Objective To describe number of patients diagnosed with influenza in Norway each season and the number treated in primary or specialist health care by age. Further, to compare the burden of seasonal influenza with the 2009‐2010 pandemic outbreak. Methods We used Norwegian national health registries and identified all patients diagnosed with influenza from 2008 to 2017. We calculated seasonal rates, compared hospitalized patients with patients in primary care and compared seasonal influenza with the 2009‐2010 pandemic outbreak. Results Each season, on average 1.7% of the population were diagnosed with influenza in primary care, the average rate of hospitalization was 48 per 100 000 population while the average number of hospitalized patients each season was nearly 2500. The number of hospitalized influenza patients ranged from 579 in 2008‐2009 to 4973 in 2016‐2017. Rates in primary care were highest among young adults while hospitalization rates were highest in patients 80 years and older and in children below 5 years. The majority of in‐hospital deaths were in patients 70 years and older. Fewer patients were hospitalized during the 2009‐2010 pandemic than in seasonal outbreaks, but during the pandemic, more people in the younger age groups were hospitalized and fatal cases were younger. Conclusion Influenza causes a substantial burden in primary care and hospitals. In non‐pandemic seasons, people above 80 years have the highest risk of influenza hospitalization and death.

• Haugnes, Håvard; Elstrøm, Petter; Kacelnik, Oliver; Jadczak, Urszula Agnieszka; Wisløff, Torbjørn & Blasio, Birgitte Freiesleben de (2019). Financial and Temporal Costs of Patient Isolation in Norwegian Hospitals. Journal of Hospital Infection. ISSN 0195-6701. doi: 10.1016/j.jhin.2019.11.012. Full text in Research Archive
• Engebretsen, Solveig; Engø-Monsen, Kenth; Rattalma, Arnoldo Frigessi Di & Blasio, Birgitte Freiesleben De (2019). A theoretical single-parameter model for urbanisation to study infectious disease spread and interventions. PLoS Computational Biology. ISSN 1553-734X. 15(3). doi: 10.1371/journal.pcbi.1006879. Full text in Research Archive Show summary

  The world is continuously urbanising, resulting in clusters of densely populated urban areas and more sparsely populated rural areas. We propose a method for generating spatial fields with controllable levels of clustering of the population. We build a synthetic country, and use this method to generate versions of the country with different clustering levels. Combined with a metapopulation model for infectious disease spread, this allows us to in silico explore how urbanisation affects infectious disease spread. In a baseline scenario with no interventions, the underlying population clustering seems to have little effect on the final size and timing of the epidemic. Under within-country restrictions on non-commuting travel, the final size decreases with increased population clustering. The effect of travel restrictions on reducing the final size is larger with higher clustering. The reduction is larger in the more rural areas. Within-country travel restrictions delay the epidemic, and the delay is largest for lower clustering levels. We implemented three different vaccination strategies—uniform vaccination (in space), preferentially vaccinating urban locations and preferentially vaccinating rural locations. The urban and uniform vaccination strategies were most effective in reducing the final size, while the rural vaccination strategy was clearly inferior. Visual inspection of some European countries shows that many countries already have high population clustering. In the future, they will likely become even more clustered. Hence, according to our model, within-country travel restrictions are likely to be less and less effective in delaying epidemics, while they will be more effective in decreasing final sizes. In addition, to minimise final sizes, it is important not to neglect urban locations when distributing vaccines. To our knowledge, this is the first study to systematically investigate the effect of urbanisation on infectious disease spread and in particular, to examine effectiveness of prevention measures as a function of urbanisation.

• Randjelovic, Ivana; Moghaddam, Amir; Blasio, Birgitte Freiesleben De & Moi, Harald (2018). The role of polymorphonuclear leukocyte counts from urethra, cervix, and vaginal wet mount in diagnosis of nongonococcal lower genital tract infection. Infectious Diseases in Obstetrics and Gynecology. ISSN 1064-7449. 2018. doi: 10.1155/2018/8236575. Full text in Research Archive Show summary

  Objective. The aim of this study was to evaluate whether the polymorphonuclear leukocyte (PMNL) inflammatory response in women with nongonococcal lower genital tract infection (LGTI) can be used to optimize criteria for syndromic treatment. Methods. A cross-sectional study of 375 women attending the STI clinic in Oslo. Urethral, cervical, and vaginal specimens underwent microscopy for PMNLs. Chlamydia trachomatis (Ct) and other STIs were detected in the cervical/vaginal swabs and urine, using nucleic acid amplification test (NAAT). After excluding vulvovaginal candidiasis, genital herpes, and trichomoniasis, we correlated clinical and microscopic signs of inflammation with positive NAAT for Ct, mycoplasma genitalium (Mg), and Ureaplasma urealyticum (Uu) in a subgroup of 293 women. Results. To predict a positive Ct, the combination of high cut-off urethritis (≥10 PMNLs/HPF) and microscopic cervicitis had a high specificity of 0.93, a PPV of 0.37, and a sensitivity of 0.35. LGTI criteria had low predicting values for Mg and Uu. Conclusion. Including microscopic criteria for the diagnosis of LGTI gives better indication for presumptive antibiotic treatment than anamnestic and clinical diagnosis alone.

• Ruscio, Francesco Di; Bjørnholt, Jørgen; Larssen, Kjersti Wik; Leegaard, Truls Michael; Moen, Aina Elisabeth Fossum & Blasio, Birgitte Freiesleben De (2018). Epidemiology and spa-type diversity of meticillin-resistant Staphylococcus aureus in community and healthcare settings in Norway. Journal of Hospital Infection. ISSN 0195-6701. doi: 10.1016/j.jhin.2017.12.019. Full text in Research Archive Show summary

  Background: There has been a marked increase in the incidence of meticillin-resistant Staphylococcus aureus (MRSA) during the past decade in Norway; a country with one of the lowest prevalence rates and an active ‘search-and-destroy’ policy applied to hospital settings. Aim: To characterize the trends of notification rates of community-associated (CA) and healthcare-associated (HA) MRSA in Norway, and explore the diversity and circulation of MRSA spa types within and outside healthcare settings. Methods: A registry-based study on notified MRSA infections and colonizations was conducted in Norway between 2006 and 2015. The diversity and abundance of CA- and HA-MRSA spa types were compared using novel ecological diversity measures (Hill numbers). Findings: During the study period, the monthly notification rate increased 6.9-fold and 1.8-fold among CA- and HA-MRSA, respectively; the increase was steeper among colonizations than infections. In both settings, the distribution of spa types was uneven, with a few dominant spa types and many singletons. The spa-type diversity of CA-MRSA was higher than HA-MRSA in terms of different types (685 vs 481), and increased during the study period. However, the diversity associated with the dominant spa types was similar and remained stable. A high overlap of spa types was estimated between the settings; spa-t002, t019 and t008 were the most common. Conclusion: The present findings suggest a strong connection between CA- and HA-MRSA epidemiology in Norway. If the fast-growing trend of CA-MRSA continues in the years to come, it may challenge current guidelines and infection control of MRSA in healthcare environments.

• Engebretsen, Solveig; Frigessi, Arnoldo; Engø-Monsen, Kenth; Furberg, Anne-Sofie; Stubhaug, Audun & Blasio, Birgitte Freiesleben De [Show all 7 contributors for this article] (2018). The peer effect on pain tolerance. Scandinavian Journal of Pain. ISSN 1877-8860. 18(3), p. 467–477. doi: 10.1515/sjpain-2018-0060. Full text in Research Archive Show summary

  Background and aims:<br> Twin studies have found that approximately half of the variance in pain tolerance can be explained by genetic factors, while shared family environment has a negligible effect. Hence, a large proportion of the variance in pain tolerance is explained by the (non-shared) unique environment. The social environment beyond the family is a potential candidate for explaining some of the variance in pain tolerance. Numerous individual traits have previously shown to be associated with friendship ties. In this study, we investigate whether pain tolerance is associated with friendship ties. <br> Methods:<br> We study the friendship effect on pain tolerance by considering data from the Tromsø Study: Fit Futures I, which contains pain tolerance measurements and social network information for adolescents attending first year of upper secondary school in the Tromsø area in Northern Norway. Pain tolerance was measured with the cold-pressor test (primary outcome), contact heat and pressure algometry. We analyse the data by using statistical methods from social network analysis. Specifically, we compute pairwise correlations in pain tolerance among friends. We also fit network autocorrelation models to the data, where the pain tolerance of an individual is explained by (among other factors) the average pain tolerance of the individual’s friends. <br> Results: <br> We find a significant and positive relationship between the pain tolerance of an individual and the pain tolerance of their friends. The estimated effect is that for every 1 s increase in friends’ average cold-pressor tolerance time, the expected cold-pressor pain tolerance of the individual increases by 0.21 s (p-value: 0.0049, sample size n=997). This estimated effect is controlled for sex. The friendship effect remains significant when controlling for potential confounders such as lifestyle factors and test sequence among the students. Further investigating the role of sex on this friendship effect, we only find a significant peer effect of male friends on males, while there is no significant effect of friends’ average pain tolerance on females in stratified analyses. Similar, but somewhat lower estimates were obtained for the other pain modalities. <br> Conclusions:<br> We find a positive and significant peer effect in pain tolerance. Hence, there is a significant tendency for students to be friends with others with similar pain tolerance. Sex-stratified analyses show that the only significant effect is the effect of male friends on males. <br> Implications:<br> Two different processes can explain the friendship effect in pain tolerance, selection and social transmission. Individuals might select friends directly due to similarity in pain tolerance, or indirectly through similarity in other confounding variables that affect pain tolerance. Alternatively, there is an influence effect among friends either directly in pain tolerance, or indirectly through other variables that affect pain tolerance. If there is indeed a social influence effect in pain tolerance, then the social environment can account for some of the unique environmental variance in pain tolerance. If so, it is possible to therapeutically affect pain tolerance through alteration of the social environment.

• Morris, Sinead E.; Blasio, Birgitte Freiesleben De; Viboud, Cécile; Wesolowski, Amy; Bjørnstad, Ottar Nordal & Grenfell, Bryan T. (2018). Analysis of multi-level spatial data reveals strong synchrony in seasonal influenza epidemics across Norway, Sweden, and Denmark. PLOS ONE. 13(5). doi: 10.1371/journal.pone.0197519. Full text in Research Archive Show summary

  Population structure, spatial diffusion, and climatic conditions mediate the spatiotemporal spread of seasonal influenza in temperate regions. However, much of our knowledge of these dynamics stems from a few well-studied countries, such as the United States (US), and the extent to which this applies in different demographic and climatic environments is not fully understood. Using novel data from Norway, Sweden, and Denmark, we applied wavelet analysis and non-parametric spatial statistics to explore the spatiotemporal dynamics of influenza transmission at regional and international scales. We found the timing and amplitude of epidemics were highly synchronized both within and between countries, despite the geographical isolation of many areas in our study. Within Norway, this synchrony was most strongly modulated by population size, confirming previous findings that hierarchical spread between larger populations underlies seasonal influenza dynamics at regional levels. However, we found no such association when comparing across countries, suggesting that other factors become important at the international scale. Finally, to frame our results within a wider global context, we compared our findings from Norway to those from the US. After correcting for differences in geographic scale, we unexpectedly found higher levels of synchrony in Norway, despite its smaller population size. We hypothesize that this greater synchrony may be driven by more favorable and spatially uniform climatic conditions, although there are other likely factors we were unable to consider (such as reduced variation in school term times and differences in population movements). Overall, our results highlight the importance of comparing influenza spread at different spatial scales and across diverse geographic regions in order to better understand the complex mechanisms underlying disease dynamics.

• Meijerink, Hinta; White, Richard; Løvlie, Astrid Louise; Blasio, Birgitte Freiesleben De; Dalgard, Olav & Amundsen, Ellen Johanna [Show all 8 contributors for this article] (2017). Modelling the burden of hepatitis C infection among people who inject drugs in Norway, 1973–2030. BMC Infectious Diseases. 17. doi: 10.1186/s12879-017-2631-2. Full text in Research Archive Show summary

  Background Lack of Hepatitis C virus (HCV) incidence data in (Norwegian) high-risk groups impedes the ability to make informed decisions on prevention measures. Thus we rely on modelling to estimate the incidence and burden of HCV infections. Methods We constructed a compartmental model for HCV infections in Norway among active and former people who inject drugs (PWIDs). We based yearly transition rates on literature. The model was fitted to absolute numbers of hepatitis C associated cirrhosis, hepatocellular carcinoma (HCC) and death from national data sources (2000–2013). We estimated the number (95%CI) of HCV infections, cirrhosis, HCC and death and disability adjusted life years (DALYs) due to HCV infections in Norway, 1973–2030. We assumed treatment rates in the projected period were similar to those in 2013. Results The estimated proportion of chronic HCV (including those with cirrhosis and HCC) among PWIDs was stable from 2000 (49%; 4441/9108) to 2013 (43%; 3667/8587). We estimated that the incidence of HCV among PWIDs was 381 new infections in 2015. The estimated number of people with cirrhosis, HCC, and liver transplant was predicted to increase until 2022 (1537 people). DALYs among active PWIDs estimated to peak in 2006 (3480 DALYs) and decrease to 1870 DALYs in 2030. Chronic HCV infection contributes most to the total burden of HCV infection, and peaks at 1917 DALYs (52%) in 2007. The burden of HCV related to PWID increased until 2006 with 81/100,000 DALYs inhabitants and decreased to 68/100,000 DALYs in 2015. Conclusion The burden of HCV associated with injecting drug use is considerable, with chronic HCV infection contributing most to the total burden. This model can be used to estimate the impact of different interventions on the HCV burden in Norway and to perform cost-benefit analyses of various public health measures.

• Marangi, Luigi; Mirinaviciute, Grazina; Flem, Elmira; Tomba, Gianpaolo Scalia; Guzzetta, Giorgio & Blasio, Birgitte Freiesleben De [Show all 7 contributors for this article] (2017). The natural history of varicella zoster virus infection in Norway: Further insights on exogenous boosting and progressive immunity to herpes zoster. PLOS ONE. 12(5). doi: 10.1371/journal.pone.0176845. Full text in Research Archive Show summary

  We use age-structured models for VZV transmission and reactivation to reconstruct the natural history of VZV in Norway based on available pre-vaccination serological data, contact matrices, and herpes zoster incidence data. Depending on the hypotheses on contact and transmission patterns, the basic reproduction number of varicella in Norway ranges between 3.7 and 5.0, implying a vaccine coverage between 73 and 80% to effectively interrupt transmission with a 100% vaccine efficacy against infection. The varicella force of infection peaks during early childhood (3–5 yrs) and shows a prolonged phase of higher risk during the childbearing period, though quantitative variations can occur depending on contact patterns. By expressing the magnitude of exogenous boosting as a proportion of the force of infection, it is shown that reactivation is well described by a progressive immunity mechanism sustained by a large, though possibly below 100%, degree of exogenous boosting, in agreement with findings from other Nordic countries, implying large reproduction numbers of boosting. Moreover, magnitudes of exogenous boosting below 40% are robustly disconfirmed by data. These results bring further insight on the magnitude of immunity boosting and its relationship with reactivation.

• Andreassen, A Elizabeth S; Jacobsen, Caroline M; Blasio, Birgitte Freiesleben De; White, Richard; Kristiansen, Ivar Sønbø & Elstrøm, Petter (2017). The impact of methicillin-resistant S. aureus on length of stay, readmissions and costs: a register based case-control study of patients hospitalized in Norway. Antimicrobial Resistance & Infection Control. 6. doi: 10.1186/s13756-017-0232-x. Full text in Research Archive Show summary

  Background: Patients with methicillin-resistant S. aureus (MRSA) are thought to incur additional costs for hospitals due to longer stay and contact isolation. The aim of this study was to assess the costs associated with MRSA in Norwegian hospitals. Methods: Analyses were based on data fromSouth-Eastern Norway for the year 2012 as registered in the Norwegian Surveillance System for Communicable Diseases and the Norwegian Patient Registry. We used a matched case-control method to compare MRSA diagnosed inpatients with non-MRSA inpatients in terms of length of stay, readmissions within 30 days from discharge, as well as the Diagnosis-Related Group (DRG) based costs. Results: Norwegian patients with MRSA stayed on average 8 days longer in hospital than controls, corresponding to a ratio of mean duration of 2.08 (CI 95%, 1.75–2.47) times longer.A total of 14% of MRSA positive inpatients were readmitted compared to 10% among controls. However, the risk of readmission was not significantly higher for patients with MRSA. DRG based hospital costs were 0.37 (95% CI, 0.19–0.54) times higher among cases than controls, with a mean cost of EUR13,233(SD 26,899) and EUR7198(SD 18,159) respectively. Conclusion: The results of this study indicate that Norwegian patients with MRSA have longer hospital stays, and higher costs than those without MRSA.

• Ruscio, Francesco Di; Bjørnholt, Jørgen; Leegaard, Truls Michael; Moen, Aina Elisabeth Fossum & Blasio, Birgitte Freiesleben De (2017). MRSA infections in Norway: A study of the temporal evolution, 2006-2015. PLOS ONE. 12(6). doi: 10.1371/journal.pone.0179771. Full text in Research Archive Show summary

  Background: Norway has one of the lowest prevalences of methicillin-resistant Staphylococcus aureus (MRSA) infections in the world. This study exploits the extensive data on MRSA infections in the Norwegian surveillance system to investigate the important factors defining the MRSA epidemiology. Methods: We performed a quasi-Poisson regression of the monthly notification rate (NR) of MRSA infections reported from January 2006 to December 2015, comparing the time trend among people with an immigrant vs. Norwegian background and domestic vs. imported infections, stratified by age groups. Findings: A total of 5289 MRSA infections were reported during the study period, of which 2255 (42·6%) were acquired in Norway, 1370 (25·9%) abroad, and 1664 (31·5%) with an unknown place of acquisition. Overall, the monthly NR increased significantly from 2006 to 2015 (+0·8% each month). The monthly increase in immigrants (+1·3%) was steeper than that in people with a Norwegian background (+0·6%). There was a significant growth (+0·4%) in the rate of domestically acquired infections, however, the NR of infections acquired abroad increased faster (+0·8%). For both imported and domestic infections, the increase occurred in persons aged < 70 years. Interpretation: Our analysis suggests that immigration and importation, especially among persons aged < 40 years, represent important factors for the increasing notification rate of MRSA infections in Norway.

• Edwards, Christina; Blasio, Birgitte Freiesleben De; Salamanca, Beatriz Valcarcel & Flem, Elmira (2017). Re–evaluation of the cost–effectiveness and effects of childhood rotavirus vaccination in Norway. PLOS ONE. 12(8). doi: 10.1371/journal.pone.0183306. Full text in Research Archive Show summary

  Background: Rotavirus vaccination was included into the Norwegian childhood immunisation programme in 2014. Before implementation, rotavirus vaccination was found to be cost–effective from a societal perspective, but not from a healthcare perspective. Since introduction, new data on the incidence and economic effects of rotavirus disease have become available. We assessed early epidemiological effects of the rotavirus vaccination programme and re–evaluated its cost–effectiveness in Norway for the years 2015–2019. Methods: Using a dynamic transmission model, we compared the epidemiological effects of the ongoing two–dose vaccination programme with Rotarix®, and a hypothetical 3–dose programme with RotaTeq® with no vaccination. A baseline cost of € 54 per fully vaccinated child was used. Cost–effectiveness was computed from a healthcare and societal perspective, using a decision analytical model. Data on healthcare use and costs, productivity losses and health utilities were based on published and own estimates. Uncertainty was accounted for in one–way, multi–way, and probabilistic sensitivity analyses. Results: During 2015–2019, 114,658 home care cases, 34,571 primary care cases, 7,381 severe cases, and 2 deaths associated with rotavirus disease were avoided due to vaccination. Under baseline assumptions vaccination was cost–effective from a healthcare perspective with a cost per QALY of € 47,447 for Rotarix® and € 52,709 for RotaTeq®. The break–even price was € 70 for Rotarix® and € 67 for RotaTeq®. Vaccination was cost–saving from the societal perspective, and also from a healthcare perspective for vaccine prices below € 25 and € 22 per vaccinated child for Rotarix® and RotaTeq®, respectively.

• Brisson, Marc; Bénard, Élodie; Drolet, Mélanie; Bogaards, Johannes A; Baussano, Iacopo & Vänskä, Simopekka [Show all 31 contributors for this article] (2016). Population-level impact, herd immunity, and elimination after human papillomavirus vaccination: a systematic review and meta-analysis of predictions from transmission-dynamic models. The Lancet Public Health. ISSN 2468-2667. 1(1), p. e8–e17. doi: 10.1016/S2468-2667(16)30001-9. Full text in Research Archive
• Herrador, Bernardo Rafael Guzmán; Blasio, Birgitte Freiesleben De; Lund, Vidar; MacDonald, Emily; Vold, Line & Wahl, Erik [Show all 7 contributors for this article] (2016). Vannbårne utbrudd i Norge i perioden 2003–12 :. Tidsskrift for Den norske legeforening (Tidsskriftet). ISSN 0029-2001. 136(7), p. 612–616. doi: 10.4045/tidsskr.15.0114. Full text in Research Archive
• Herrador, Bernardo Rafael Guzmán; Blasio, Birgitte Freiesleben De; Carlander, Anneli; Ethelberg, Steen; Hygen, Hans Olav & Kuusi, Markku [Show all 17 contributors for this article] (2016). Association between heavy precipitation events and waterborne outbreaks in four Nordic countries, 1992-2012. Journal of Water and Health. ISSN 1477-8920. 14(6), p. 1019–1027. doi: 10.2166/wh.2016.071. Full text in Research Archive
• Fava, Emanuele Del; Rimseliene, Grazina; Flem, Elmira; Blasio, Birgitte Freiesleben De; Tomba, Gianpaolo Scalia & Manfredi, Piero (2016). Estimating age-specific immunity and force of infection of varicella zoster virus in Norway using mixture models. PLOS ONE. 11(9). doi: 10.1371/journal.pone.0163636. Full text in Research Archive Show summary

  This study applies mixture modelling to examine age-specific immunity to varicella zoster virus (VZV) infection in Norway based on the first large-scale serological study in the general population. We estimated the seropositive proportions at different ages and calculated the underlying force of infection by using a sample of 2103 residual sera obtained from patients seeking primary and hospital care. A rapid increase in the VZV-associated immunity is observed in the first years of life with 63% of children being immune by age 5. The increase in the immunity levels slows down thereafter, with a large proportion of adults still susceptible by age 20 (around 14.5%), thus at risk of serious sequelae of varicella infection. The corresponding force of infection peaks during the preschool period, subsequently declines to a minimum between ages 10 and 20 years, and afterwards moderately increases to reach a plateau lasting throughout the childbearing period. In comparison with the traditional cut-off approach, mixture modelling used the whole data without producing any inconclusive cases, led to an unbiased classification of individuals between susceptible and immune, and provided a smoother immune profile by age. These findings represent an important step towards any decision about the introduction of varicella vaccination in Norway, as they are a primary input for mathematical transmission models aimed at evaluating potential vaccination scenarios.

• Molin, Sol-Britt; Blasio, Birgitte Freiesleben De & Olsen, Anne Olaug (2016). Is the risk for sexually transmissible infections (STI) lower among women with exclusively female sexual partners compared with women with male partners? A retrospective study based on attendees at a Norwegian STI clinic from 2004 to 2014. Sexual Health. ISSN 1448-5028. 13(3), p. 257–264. doi: 10.1071/SH15193. Full text in Research Archive
• Edwards, Christina Hansen; Tomba, Gianpaolo Scalia & Blasio, Birgitte Freiesleben De (2016). Influenza in workplaces: Transmission, workers' adherence to sick leave advice and European sick leave recommendations. European Journal of Public Health. ISSN 1101-1262. 26(3), p. 478–485. doi: 10.1093/eurpub/ckw031. Full text in Research Archive
• Herrador, Bernardo Rafael Guzmán; Blasio, Birgitte Freiesleben De; MacDonald, Emily; Nichols, Gordon; Sudre, Bertrand & Vold, Line [Show all 8 contributors for this article] (2015). Analytical studies assessing the association between extreme precipitation or temperature and drinking water-related waterborne infections: a review. Environmental health. 14. doi: 10.1186/s12940-015-0014-y. Full text in Research Archive Show summary

  Determining the role of weather in waterborne infections is a priority public health research issue as climate change is predicted to increase the frequency of extreme precipitation and temperature events. To document the current knowledge on this topic, we performed a literature review of analytical research studies that have combined epidemiological and meteorological data in order to analyze associations between extreme precipitation or temperature and waterborne disease. A search of the databases Ovid MEDLINE, EMBASE, SCOPUS and Web of Science was conducted, using search terms related to waterborne infections and precipitation or temperature. Results were limited to studies published in English between January 2001 and December 2013. Twenty-four articles were included in this review, predominantly from Asia and North-America. Four articles used waterborne outbreaks as study units, while the remaining articles used number of cases of waterborne infections. Results presented in the different articles were heterogeneous. Although most of the studies identified a positive association between increased precipitation or temperature and infection, there were several in which this association was not evidenced. A number of articles also identified an association between decreased precipitation and infections. This highlights the complex relationship between precipitation or temperature driven transmission and waterborne disease. We encourage researchers to conduct studies examining potential effect modifiers, such as the specific type of microorganism, geographical region, season, type of water supply, water source or water treatment, in order to assess how they modulate the relationship between heavy rain events or temperature and waterborne disease. Addressing these gaps is of primary importance in order to identify the areas where action is needed to minimize negative impact of climate change on health in the future.

• Herrador, Bernardo Rafael Guzmán; Carlander, Anneli; Ethelberg, Steen; Blasio, Birgitte Freiesleben De; Kuusi, Markku & Lund, Vidar [Show all 15 contributors for this article] (2015). Waterborne outbreaks in the Nordic countries, 1998 to 2012. Eurosurveillance. ISSN 1025-496X. 20(24). doi: 10.2807/1560-7917.ES2015.20.24.21160. Full text in Research Archive
• White, Richard; MacDonald, Emily; Blasio, Birgitte Freiesleben De; Nygård, Karin Maria; Vold, Line & Røttingen, John-Arne (2015). Projected Treatment Capacity Needs in Sierra Leone. PLOS Currents. 7. doi: 10.1371/currents.outbreaks.3c3477556808e44cf41d2511b21dc29f. Full text in Research Archive
• Steens, Anneke; VESTRHEIM, DIDRIK FRIMANN & Blasio, Birgitte Freiesleben De (2015). Pneumococcal vaccination in older adults in the era of childhood vaccination: Public health insights from a Norwegian statistical prediction study. Epidemics. ISSN 1755-4365. 11, p. 24–31. doi: 10.1016/j.epidem.2015.01.001. Full text in Research Archive Show summary

  Two different vaccines, a 23-valent polysaccharide vaccine (PPV23) and a 13-valent conjugate vaccine (PCV13), are available for prevention of invasive pneumococcal disease (IPD) in the population aged 65 years and older (65+). The IPD epidemiology in the 65+ is undergoing change due to indirect effects of childhood immunisation. Vaccine recommendations for the 65+ must take into account these trends in epidemiology. We therefore explored the preventive potential of vaccination strategies to prevent IPD in the 65+, including PPV23, PCV13 or PCV13 + PPV23 in 2014–2019. Quasi-Poisson regression models were fitted to 2004–2014 population-wide surveillance data and used to predict incidences for vaccine-type and non-vaccine type IPD. We determined the number of people needed to be vaccinated to prevent one case per season (NNV) for each strategy and estimated the public health impact on the IPD case counts from increasing the vaccine uptake to 28–45%. Our results indicate that PCV13-IPD will decrease by 71% from 58 (95% prediction interval 55–61) cases in 2014/15 to 17 (6–52) in 2018/19 and PPV23-IPD by 32% from 168 (162–175) to 115 (49–313) cases. The NNV will increase over time for all strategies because of a decreasing vaccine-type IPD incidence. In 2018/19, the PCV13-NNV will be 5.3 times higher than the PPV23-NNV. Increasing the vaccine uptake will lead to a larger public health impact for all scenarios. Combining PCV13 and PPV23 is most effective, but the additional effect of PCV13 will decrease and is only marginal in 2018/19. Our study demonstrates the importance of increasing PPV23 uptake and of developing vaccines that confer broader immunity.

• Blasio, Fabio V De; Liow, Lee Hsiang; Schweder, Tore & Blasio, Birgitte Freiesleben De (2015). A model for global diversity in response to temperature change over geological time scales, with reference to planktic organisms. Journal of Theoretical Biology. ISSN 0022-5193. 365, p. 445–456. doi: 10.1016/j.jtbi.2014.10.031. Full text in Research Archive
• Blasio, Birgitte Freiesleben De; Flem, Elmira; Latipov, Renat; Kuatbaeva, Ajnagul & Kristiansen, Ivar Sønbø (2014). Dynamic Modeling of Cost-effectiveness of Rotavirus Vaccination, Kazakhstan. Emerging Infectious Diseases. ISSN 1080-6040. 20(1), p. 29–37. doi: 10.3201/eid2001.130019. Full text in Research Archive
• Mdala, Ibrahimu; Olsen, Ingar; Haffajee, Anne D; Socransky, Sigmund S; Thoresen, Magne & Blasio, Birgitte Freiesleben De (2014). Comparing clinical attachment level and pocket depth for predicting periodontal disease progression in healthy sites of patients with chronic periodontitis using multi-state Markov models. Journal of Clinical Periodontology. ISSN 0303-6979. 41(9), p. 837–845. doi: 10.1111/jcpe.12278. Full text in Research Archive
• Mdala, Ibrahimu; Olsen, Ingar; Haffajee, Anne D; Socransky, Sigmund S; Thoresen, Magne & Blasio, Birgitte Freiesleben De (2013). Kan vi forutsi områder i periodontiet som vil utvikle periodontitt? Den norske tannlegeforenings Tidende. ISSN 0029-2303. 123(8), p. 556–557. Full text in Research Archive

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• Storvik, Geir Olve; Engebretsen, Solveig; Blasio, Birgitte Freiesleben De & Frigessi, Arnoldo (2025). Flaws in the Article “Nearly Instantaneous Time-Varying Reproduction Number for Contagious Diseases—a Direct Approach Based on Nonlinear Regression". Journal of Computational Biology. ISSN 1066-5277. 32(8), p. 813–818. doi: https:/www.liebertpub.com/doi/full/10.1177/15578666251360613. Full text in Research Archive
• Engebretsen, Solveig; Thorarinsdottir, Thordis Linda; Palomares, Alfonso Diz-Lois; Storvik, Geir Olve; Frigessi, Arnoldo & Blasio, Birgitte Freiesleben De (2025). Contribution to the Discussion of 'Some statistical aspects of the Covid-19 response' by Wood et al. Journal of the Royal Statistical Society, Series A (Statistics in Society). ISSN 0964-1998. 189(1), p. 71–72. doi: 10.1093/jrsssa/qnaf080. Full text in Research Archive
• Midtbø, Jørgen E.; Blasio, Birgitte Freiesleben De; Ruscio, Francesco Di; Chan, Yat Hin; Stoltenberg, Camilla & Vold, Line [Show all 7 contributors for this article] (2022). Nei, eit reknestykke held ikkje Noreg stengt. Full text in Research Archive
• Blasio, Birgitte Freiesleben De (2022). “Use of real-time mobility in modelling the COVID-19 pandemic: The Norwegian experience”. Full text in Research Archive
• Blasio, Birgitte Freiesleben de & Lindman, Anja Elsrud Schou (2021). Fra krøttersti til motorvei - et beredskapsregister ser dagens lys og gir tilgang til norske sanntidsdata for tidens mest omtalte bogstav -R. Full text in Research Archive
• Blasio, Birgitte Freiesleben de (2021). Covid-19 modelling in Norway: Preparedness, planning and response. Full text in Research Archive
• Blasio, Birgitte Freiesleben de (2021). Modellering av covid-19 pandemien i Norge - vaksinering og veien videre mot gjenåpning. Full text in Research Archive
• Stoltenberg, Camilla; Aavitsland, Preben; Rø, Gunnar Øyvind Isaksson; Blasio, Birgitte Freiesleben de; Rattalma, Arnoldo Frigessi Di & Engebretsen, Solveig (2021). Aftenposten bommer om prognoser. Igjen og igjen. Full text in Research Archive
• Blasio, Birgitte Freiesleben de (2021). A real-time covid-19 modelling pipeline for Norway. Full text in Research Archive
• Blasio, Birgitte Freiesleben de (2021). Matematisk modellering av SARS-CoV-2 pandemien - hva kan vi vente oss framover? Full text in Research Archive
• Hauge, Siri Helene; Bakken, Inger Johanne Landsjøåsen; Blasio, Birgitte Freiesleben De & Håberg, Siri Eldevik (2021). Correction to: Risk conditions in children hospitalized with influenza in Norway, 2017–2019 (BMC Infectious Diseases, (2020), 20, 1, (769), 10.1186/s12879-020-05486-6). BMC Infectious Diseases. 21(1). doi: 10.1186/s12879-021-05913-2. Full text in Research Archive
• Bakken, Jari; Johansen, Per Anders; Sandberg, Hallvard; Skille, Øyvind Bye; Blasio, Birgitte Freiesleben De & Engebretsen, Solveig [Show all 8 contributors for this article] (2021). Panel debate: Mathematical models, statistical complexity, uncertainty and decision making in the Covid-19 pandemic: science, society and the public. A panel discussion with journalists and modellers. [Internet]. Big Insight Day 2021.. Full text in Research Archive
• Blasio, Birgitte Freiesleben De (2020). COVID-19 modellering ved FHI. Full text in Research Archive
• Blasio, Birgitte Freiesleben De & Svaar, Peter (2020). Nye FHI-beregninger: Viruset er mindre utbredt, men mer dødelig. [Internet]. NRK. Full text in Research Archive
• Blasio, Birgitte Freiesleben De & Christian, Birk (2020). Mens danske myndigheder mørklægger centrale beregninger om coronavirussen, er stilen en helt anden i Norge og Sverige: »Det mener jeg, er den rigtige måde at gøre det på«. [Internet]. Berlingske.dk. Full text in Research Archive
• Borgan, Eldrid & Blasio, Birgitte Freiesleben De (2020). Slik regner FHI-forskerne på hva som er lurt å gjøre hvis koronasmitten blusser opp igjen. [Journal]. Forskning.no. Full text in Research Archive
• Blasio, Birgitte Freiesleben De (2020). NRK-nyheter intervju: Mobilsporingen de håper skal hjelpe mot koronaviruset. [TV]. NRK. Full text in Research Archive
• Kristiansen, Ivar Sønbø; Burger, Emily & Blasio, Birgitte Freiesleben De (2020). Covid-19: Simuleringsmodeller ved epidemier. 140(6), p. 1–6. doi: 10.4045/TIDSSKR.20.0225. Full text in Research Archive
• Blasio, Birgitte Freiesleben De (2020). Modellering av COVID-19 epidemien i Norge. Full text in Research Archive
• Blasio, Birgitte Freiesleben de (2020). Modellering av covid-19 epidemien i Norge, Tillit i koronaens tid. Dreyer Forlag A/S. ISSN 9788282655569. p. 51–74. Full text in Research Archive
• Blasio, Birgitte Freiesleben De & Osman, Iqbal (2020). BBC World interview. [TV]. BBC. Full text in Research Archive
• Blasio, Birgitte Freiesleben De (2020). Status of the COVID-19 pandemic globally and in Norway. Full text in Research Archive
• Lydersen, Stian & Blasio, Birgitte Freiesleben De (2020). Hva er flokkimmunitet? Tidsskrift for Den norske legeforening (Tidsskriftet). ISSN 0029-2001. doi: 10.4045/tidsskr.20.0898. Full text in Research Archive
• Kristiansen, Ivar Sønbø; Burger, Emily & Blasio, Birgitte Freiesleben De (2020). Covid-19: Simuleringsmodeller ved epidemier. 140(6). doi: 10.4045/tidsskr.20.0225. Full text in Research Archive
• Blasio, Birgitte Freiesleben De (2018). Rotavirusvaksinasjon i Norge er kostnadseffektiv. BestPractice Infeksjon og vaksinasjon. Full text in Research Archive
• Edwards, Christina Hansen; Blasio, Birgitte Freiesleben de; Salamanca, Beatriz Valcarcel & Flem, Elmira (2016). Is ongoing rotavirus vaccination in Norway cost-effective? Full text in Research Archive
• Blasio, Birgitte Freiesleben De (2014). Et Innblikk i infeksjonsmodellering--med eksempler. Full text in Research Archive
• Edwards, Christina Hansen; Tomba, Gianpaolo Scalia & Blasio, Birgitte Freiesleben De (2014). Recommendations and policies for sick leave during seasonal and pandemic influenza in Europe. Full text in Research Archive
• Edwards, Christina Hansen; Tomba, Gianpaolo Scalia & Blasio, Birgitte Freiesleben De (2014). Dynamic modeling of the cost-effectiveness of sick leave practices during influenza illness. Full text in Research Archive
• Zechmeister, Ingrid; Blasio, Birgitte Freiesleben De; Garnett, Geoffrey; Neilson, Aileen Rae & Siebert, Uwe (2014). Corrigendum to: Cost-effectiveness analysis of human papillomavirus-vaccination programs to prevent cervical cancer in Austria [Vaccine 27 (2009) 5133-5141]. Vaccine. ISSN 0264-410X. 32(21), p. 2520–2520. doi: 10.1016/j.vaccine.2014.03.006. Full text in Research Archive
• Edwards, Christina Hansen; Tomba, Gianpaolo Scalia; White, Richard Aubrey; Kristiansen, Ivar Sønbø & Blasio, Birgitte Freiesleben de (2014). Dynamic modeling of the cost-effectiveness of sick leave practices during influenza illness. Full text in Research Archive
• Tomba, Gianpaolo Scalia; Blasio, Birgitte Freiesleben de & Edwards, Christina Hansen (2014). Recommendations and policies for sick leave during seasonal and pandemic influenza in Europe. Full text in Research Archive
• Blasio, Birgitte Freiesleben De (2013). How to prepare for pandemics? Infectious disease modeling and its use in Norwegian preparedness planning. Full text in Research Archive
• Blasio, Birgitte Freiesleben De (2013). Infectious disease modeling with some examples. Full text in Research Archive
• Blasio, Birgitte Freiesleben De (2013). Influenza surveillance in Norway and Modeling of Influenza Transmission. Full text in Research Archive
• Blasio, Birgitte Freiesleben de; Ruscio, Francesco Di; Rø, Gunnar Øyvind Isaksson; Engebretsen, Solveig; Frigessi, Arnoldo & Palomares, Alfonso Diz-Lois [Show all 14 contributors for this article] (2020). Situational awareness and forecasting - FHI COVID-19 modelling team. Folkehelseinstituttet. Full text in Research Archive
• Hauge, Siri Helene; Meijerink, Hinta; Alveberg, Benedikte Louise; Berg, Are Stuwitz; Bergh, Anne & Bragstad, Karoline [Show all 28 contributors for this article] (2020). Urbanization and preparedness for outbreaks with high-impact respiratory pathogens. Folkehelseinstituttet. ISSN 9788284061078. Full text in Research Archive Show summary

  Urban areas are heterogeneous and therefore implementation of the recommendations of this report will be determined by a city’s available resources, current preparedness, as well as adjusting them to local needs and characteristics. The recommendations are based on available literature, experiences from previous outbreaks, the current COVID-19 outbreak, NIPH’s own experts and feedback from reviewers. The recommendations are given within the following areas: Multi-sectoral approach, including financial, political and governance research during an emerging outbreak Access to health care Non-pharmaceutical interventions (NPI) Infection prevention and control Outbreak surveillance Laboratory preparedness Mass vaccination plans Communicating with the population in a health crisis Modelling the outbreak Research during an emerging outbreak In addition, the report makes the following recommendations to the World Health Organization: Recommendations for urban preparedness should be developed in the years to come, including specific tools and guidelines. IHR Monitoring and Evaluation Framework should consider including urban preparedness. The Joint External Evaluations should integrate indicators that would measure subnational capacities, including in urban settings. Set up collaboration networks between megacities to share experiences, data and tools in order to prepare preparedness plans.

• Hungnes, Olav; Hauge, Siri Helene; Oftung, Fredrik; Rydland, Kjersti Margrethe; Rønning, Karin & Dudman, Susanne Gjeruldsen [Show all 21 contributors for this article] (2015). Influensasesongen i Norge 2014-15. Årsrapport sykdomsprogram for influensa. Nasjonalt folkehelseinstitutt, Divisjon for Smittevern. Full text in Research Archive
• Hauge, Siri Helene; Hungnes, Olav; Rydland, Kjersti Margrethe; Tønnessen, Ragnhild; Oftung, Fredrik & Mjaaland, Siri [Show all 12 contributors for this article] (2014). Influensasesongen i Norge 2013–14: årsrapport. Folkehelseinstituttet. Full text in Research Archive

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