Abstract
BackgroundInfluenza A/H3N2 has been circulating in humans since 1968, causing considerable morbidity and mortality. Although H3N2 incidence is highly seasonal, how such seasonality contributes to global phylogeographic migration dynamics has not yet been established. In this study, we incorporate time-varying migration rates in a Bayesian MCMC framework. We focus on migration within China, and to and from North-America as case studies, then expand the analysis to global communities.ResultsIncorporating seasonally varying migration rates improves the modeling of migration in our regional case studies, and also in a global context. In our global model, windows of increased immigration map to the seasonal timing of epidemic spread, while windows of increased emigration map to epidemic decline. Seasonal patterns also correlate with the probability that local lineages go extinct and fail to contribute to long term viral evolution, as measured through the trunk of the phylogeny. However, the fraction of the trunk in each community was found to be better determined by its overall human population size.ConclusionsSeasonal migration and rapid turnover within regions is sustained by the invasion of 'fertile epidemic grounds' at the end of older epidemics. Thus, the current emphasis on connectivity, including air-travel, should be complemented with a better understanding of the conditions and timing required for successful establishment. Models which account for migration seasonality will improve our understanding of the seasonal drivers of influenza, enhance epidemiological predictions, and ameliorate vaccine updating by identifying strains that not only escape immunity but also have the seasonal opportunity to establish and spread. Further work is also needed on additional conditions that contribute to the persistence and long term evolution of influenza within the human population, such as spatial heterogeneity with respect to climate and seasonality.Electronic supplementary materialThe online version of this article (doi:10.1186/s12862-014-0272-2) contains supplementary material, which is available to authorized users.
Highlights
Influenza A/H3N2 has been circulating in humans since 1968, causing considerable morbidity and mortality
Seasonal migration to and from, and local persistence within, North America For this analysis, we are interested in measuring the local persistence of temperate climate North American (NA) H3N2 lineages, and in inferring the seasonal timing of introductions to and from the global community to NA
The likelihood of a specific realization of a migration model is marginalized across this posterior distribution of phylogenetic trees in an additional step [24], in which tree likelihood is calculated based on the tree topology, sequence collection locations, and the specified model parameters
Summary
Influenza A/H3N2 has been circulating in humans since 1968, causing considerable morbidity and mortality. On a time scale of several years, all previously circulating lineages are globally replaced by new ones, sharing a Zinder et al BMC Evolutionary Biology (2014) 14:272 single and more recent common progenitor (2–8 years) in the past [5,6,7]. The nature of this swift global turnover remains an open question in terms of both its geographical path and its underlying mechanisms
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