Determinants of the Spatiotemporal Dynamics of the 2009 H1N1 Pandemic in Europe: Implications for Real-Time Modelling

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Abstract
Influenza pandemics in the last century were characterized by successive waves and differences in impact and timing between different regions, for reasons not clearly understood. The 2009 H1N1 pandemic showed rapid global spread, but with substantial heterogeneity in timing within each hemisphere. Even within Europe substantial variation was observed, with the UK being unique in experiencing a major first wave of transmission in early summer and all other countries having a single major epidemic in the autumn/winter, with a West to East pattern of spread. Here we show that a microsimulation model, parameterised using data about H1N1pdm collected by the beginning of June 2009, explains the occurrence of two waves in UK and a single wave in the rest of Europe as a consequence of timing of H1N1pdm spread, fluxes of travels from US and Mexico, and timing of school vacations. The model provides a description of pandemic spread through Europe, depending on intra-European mobility patterns and socio-demographic structure of the European populations, which is in broad agreement with observed timing of the pandemic in different countries. Attack rates are predicted to depend on the socio-demographic structure, with age dependent attack rates broadly agreeing with available serological data. Results suggest that the observed heterogeneity can be partly explained by the between country differences in Europe: marked differences in school calendars, mobility patterns and sociodemographic structures. Moreover, higher susceptibility of children to infection played a key role in determining the epidemiology of the 2009 pandemic. Our work shows that it would have been possible to obtain a broad-brush prediction of timing of the European pandemic well before the autumn of 2009, much more difficult to achieve with simpler models or pre-pandemic parameterisation. This supports the use of models accounting for the structure of complex modern societies for giving insight to policy makers. The 2009 H1N1pdm influenza pandemic spread rapidly but heterogeneously. A notable pattern occurred in Europe, with the UK exhibiting a first wave in early summer and a second wave in autumn, while all other European countries experienced a single wave in autumn/winter, resulting in a clear West to East pattern of spread. Our study asks which factors were most responsible for this variation, and to what extent the pattern of spread was predictable from data available in the first two months of the pandemic. Providing reliable answers to these questions would reduce uncertainty and improve situational awareness for policy-makers in the future, giving clearer expectations as to the likely impact and timing of a future pandemic and the potential effectiveness of mitigation measures. We found that that heterogeneity seen in 2009 can largely be explained by marked differences in school calendars, human mobility and demography across Europe. We also conclude that much of the variation in timing of the pandemic in Europe would have been predictable on the basis of data available in early June 2009. Our work supports the use of models accounting for the structure of complex modern societies for giving insight to policy makers in future pandemics.