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Age-specific contacts and travel patterns in the spatial spread of 2009 H1N1 influenza pandemic

Confirmed cases during the early stage of the 2009 H1N1 pdm in various countries showed an age shift between importations and local transmission cases, with adults mainly responsible for seeding unaffected regions and children most frequently driving community outbreaks. We introduce a multi-host stochastic metapopulation model with two age classes to analytically address the role of a heterogeneously mixing population and its associated non-homogeneous travel behaviors on the risk of a major epidemic. We inform the model with statistics on demography, mixing and travel behavior for Europe and Mexico, and calibrate it to the 2009 H1N1 pdm early outbreak. We varied model parameters to explore the invasion conditions under different scenarios. We derive the expression for the global invasion potential of the epidemic that depends on disease transmissibility, transportation network and mobility features, demographic profile and mixing pattern. Highly assortative mixing favor the spatial containment of the epidemic, this effect being contrasted by an increase in the social activity of adults vs. children. Heterogeneity of the mobility network topology and traffic flows strongly favor the disease invasion, as also a larger fraction of children traveling. Variations in the demography and mixing habits across countries lead to heterogeneous outbreak situations. Results are compatible with the H1N1 spatial spread observed. The work illustrates the importance of age-dependent mixing profiles and mobility features in the study of the conditions for the spatial invasion of an emerging influenza pandemic. Its results allow the immediate assessment of the risk of a major epidemic for a specific scenario upon availability of data, and the evaluation of the effectiveness of public health interventions targeting specific age groups, their interactions and mobility behaviors.