Epidemiologic aspects of poliomyelitis eradication.

Mass immunization with oral poliovirus vaccine was begun in the United States in 1963, and the last outbreak of natural poliomyelitis occurred in 1972. In the subsequent 10 years, there has been only one outbreak, in 1979, due to the introduction of wild polio-virus to an underimmunized Amish population. Paradoxically, eradication occurred even though 5%-10% of the population zero to 14 years of age were unimmunized and susceptible. It is suggested that this eradication was due to two factors. First, the marked seasonality of poliovirus infections, imposed on a reduced number of susceptibles , led to wintertime fade-outs of wild poliovirus over sizable areas. Second, although reintroductions into virus-free areas did occur, they did not equal the frequency of fade-outs. This led, in turn, to an annual stepwise reduction in the number of infected states over a 10-year period, culminating in eradication. Measles makes an instructive comparison because it has more stubbornly resisted eradication. Although regional fade-outs of measles have occurred regularly during the seasonal low, measles is more frequently reintroduced because of its greater transmissibility. It is suggested that this difference in reintroduction was a critical determinant of the difference between the eradicability of measles and poliomyelitis.