Eradication of poliomyelitis in the United States.

Mass immunization with oral poliovirus vaccine (OPV) was begun in the United States in 1963, and the least natural outbreak of poliomyelitis occurred in 1972. Since immunization programs fail to reach the total population, eradication has been achieved in the presence of a residual susceptible population of at least 5% (2-5 million children under the age of 15 years). It is proposed that the fade-out of wild polioviruses is explained by their disappearance during the winter, a low point in the yearly cycle of the virus. In the post-eradication era, the continued presence of millions of susceptible children and adults presents a constant potential hazard. Every effort should be made to maintain maximal levels of immunization with oral poliovirus vaccine and to prevent the reintroduction of wild polioviruses into the United States.