Four scenarios for the relationship between WPV infections and clinical disease.

Local persistence of polio—within a state, region, or country—occurs when WPV overwinters during the off-season and the transmission chain is unbroken year-round. In contrast, local extinction and reintroduction occurs when WPV goes extinct during the off-season, breaking the chain of transmission; a new transmission chain begins when WPV is reintroduced from elsewhere. Discriminating among these scenarios is necessary for planning eradication strategies in endemic regions.
Four scenarios for the relationship between WPV infections and clinical disease.