Revealing the True Incidence of Pandemic A(H1N1)pdm09 Influenza in Finland during the First Two Seasons - An Analysis Based on a Dynamic Transmission Model

PLoS Comput Biol. 2016 Mar 24;12(3):e1004803. doi: 10.1371/journal.pcbi.1004803. eCollection 2016 Mar.

Abstract

The threat of the new pandemic influenza A(H1N1)pdm09 imposed a heavy burden on the public health system in Finland in 2009-2010. An extensive vaccination campaign was set up in the middle of the first pandemic season. However, the true number of infected individuals remains uncertain as the surveillance missed a large portion of mild infections. We constructed a transmission model to simulate the spread of influenza in the Finnish population. We used the model to analyse the two first years (2009-2011) of A(H1N1)pdm09 in Finland. Using data from the national surveillance of influenza and data on close person-to-person (social) contacts in the population, we estimated that 6% (90% credible interval 5.1 - 6.7%) of the population was infected with A(H1N1)pdm09 in the first pandemic season (2009/2010) and an additional 3% (2.5 - 3.5%) in the second season (2010/2011). Vaccination had a substantial impact in mitigating the second season. The dynamic approach allowed us to discover how the proportion of detected cases changed over the course of the epidemic. The role of time-varying reproduction number, capturing the effects of weather and changes in behaviour, was important in shaping the epidemic.

Publication types

  • Research Support, Non-U.S. Gov't

MeSH terms

  • Adolescent
  • Adult
  • Age Distribution
  • Aged
  • Aged, 80 and over
  • Child
  • Child, Preschool
  • Computer Simulation
  • Data Interpretation, Statistical
  • Female
  • Finland / epidemiology
  • Humans
  • Incidence
  • Infant
  • Infant, Newborn
  • Influenza Vaccines / therapeutic use
  • Influenza, Human / epidemiology*
  • Influenza, Human / prevention & control*
  • Male
  • Mass Vaccination / statistics & numerical data
  • Middle Aged
  • Models, Statistical*
  • Pandemics / prevention & control*
  • Pandemics / statistics & numerical data*
  • Reproducibility of Results
  • Risk Assessment / methods
  • Seasons*
  • Sensitivity and Specificity
  • Sex Distribution
  • Young Adult

Substances

  • Influenza Vaccines

Grants and funding

National Institute for Health and Welfare, Finland (www.thl.fi), funded the project and provided the dataset. MS was funded by FICS graduate school and Academy of Finland grant no. 251170. The funders had no role in study design, data analysis, decision to publish, or preparation of the manuscript.