Epidemic emergence and source-sink dynamics.

<p>(A) Annual number of infected individuals in contrast to the small number of reported cases. Annual infections were reconstructed for the US using particle filtering means. The particle filtering mean is the expected value at time <i>t</i>, given the data up to time <i>t</i>. (B) Annual infections in the US represented as the percent of the population. Reconstructed infections show an increase in infection incidence that accompanies (C) the increase in the birth rate. (D) Simulated WPV extinction probability. The probability of extinction measured as the mean annual probability of observing an extinction during the off-season (December–May). “Sink" populations are those states with a high extinction probability. (E) Simulated trough infections. Trough infections indicate the minimum number of infections during off-seasons. For each US state, the median was taken across simulations and averaged across years. “Source" populations are those that maintain a high number of infections. Panels D–E were constructed using the 500 stochastic simulations for each state. The data used to make this figure can be found in <a href="http://www.plosbiology.org/article/info:doi/10.1371/journal.pbio.1002172#pbio.1002172.s002" target="_blank">S1 Data</a>, <a href="http://www.plosbiology.org/article/info:doi/10.1371/journal.pbio.1002172#pbio.1002172.s005" target="_blank">S4 Data</a>, <a href="http://www.plosbiology.org/article/info:doi/10.1371/journal.pbio.1002172#pbio.1002172.s006" target="_blank">S5 Data</a>, <a href="http://www.plosbiology.org/article/info:doi/10.1371/journal.pbio.1002172#pbio.1002172.s007" target="_blank">S6 Data</a>, <a href="http://www.plosbiology.org/article/info:doi/10.1371/journal.pbio.1002172#pbio.1002172.s008" target="_blank">S7 Data</a>, and <a href="http://www.plosbiology.org/article/info:doi/10.1371/journal.pbio.1002172#pbio.1002172.s010" target="_blank">S9 Data</a>.