Following the emergence of a novel strain of influenza A(H1N1) in Mexico and the United States in April 2009, its epidemiology in Europe during the summer was limited to sporadic and localised outbreaks. Only the United Kingdom experienced widespread transmission declining with school holidays in late July. Using statistical modelling where applicable we explored the following causes that could explain this surprising difference in transmission dynamics: extinction by chance, differences in the susceptibility profile, age distribution of the imported cases, differences in contact patterns, mitigation strategies, school holidays and weather patterns. No single factor was able to explain the differences sufficiently. Hence an additive mixed model was used to model the country-specific weekly estimates of the effective reproductive number using the extinction probability, school holidays and weather patterns as explanatory variables. The average extinction probability, its trend and the trend in absolute humidity were found to be significantly negatively correlated with the effective reproduction number - although they could only explain about 3% of the variability in the model. By comparing the initial epidemiology of influenza A (H1N1) across different European countries, our analysis was able to uncover a possible role for the timing of importations (extinction probability), mixing patterns and the absolute humidity as underlying factors. However, much uncertainty remains. With better information on the role of these epidemiological factors, the control of influenza could be improved.
Bibliographical noteFunding Information:
PJW thanks the UK Medical Research Council for Centre funding. PYB thanks the funding from European PCRD 7 (Flumodcont) . NH has been funded by “SIMID”, a strategic basic research project funded by the Institute for the Promotion of Innovation by Science and Technology in Flanders (IWT) , project number 060081 and by the IAP research network nr P6/03 of the Belgian Government (Belgian Science Policy). The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript.
- Absolute humidity
- Statistical models
- Swine-origin influenza A H1N1 virus