Persistence mechanisms.

<p>(A) Example of simulated infections and cases for Wisconsin. Months absent of reported cases are indicated in green. During periods when the disease is absent, WPV infections are often silently transmitted in the population. In this simulation, there were two instances (indicated by arrows) when the local chain of transmission was broken and WPV went locally extinct but quickly rebounded due to reintroduction. This example illustrates the two polio persistence mechanisms observed throughout the US, which are (i) local WPV persistence via unbroken chains of transmission and (ii) WPV extinction and reintroduction. (B) Distributions of mean monthly silent infections during periods absent of reported disease. (C) Distributions of cumulative silent infections during periods absent of disease. Distributions in B and C are 10%–90% quantiles and the median, based on 500 simulations per state. Silent infections are those that occur in the absence of reported cases and highlight the unobservable dynamics of polio. (D) Simulated cases surrounding WPV extinction events. Distributions show 10%–90% quantiles and the median number of cases observed up to 6 mo preceding and 6 mo following an extinction event. Generally, fewer than 5 cases/mo are reported 2 mo to either side of an extinction event. However, it is unclear whether 5 mo, each with less than 5 cases, is a reliable signal of extinction. 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.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>, <a href="http://www.plosbiology.org/article/info:doi/10.1371/journal.pbio.1002172#pbio.1002172.s014" target="_blank">S13 Data</a>, and <a href="http://www.plosbiology.org/article/info:doi/10.1371/journal.pbio.1002172#pbio.1002172.s015" target="_blank">S14 Data</a>.