The Epidemiology of Poliomyelitis: Enigmas Surrounding Its Appearance, Epidemicity, and Disappearance1

The emergence of a debate in the US regarding poliomyelitis immunization practices makes it important to examine unresolved issues about the epidemiology of this disease. The literature suggests that poliomyelitis initially appears as a disease of preschool children, but then there is a trend toward increasing age of cases. Age differences in the case: infection ratio do not appear to have a consistent, predictable impact on overall incidence. The appearance of epidemic poliomyelitis is assumed to have resulted from a reduction in levels of maternal antibodies as booster reinfections became less common, a reduction in the frequency of antibody levels sufficient to produce cross-protection between virus types, and an increase in the average age of primary infections. The relative importance of virus virulence and of elevated infection rate in the production of epidemics remains unknown. The use of poliovirus vaccine in the US has reduced paralytic disease from an annual incidence of about 10,000 cases to 10 cases. The eradication of poliomyelitis in spite of the fact that no more than 90% of US children have been reached indicates that either vaccine virus has spread to the unimmunized or a herd immunity effect is operating and the resultant disappearance of wild poliovirus prevents exposure of the unimmunized. The unexpected eradication of natural poliomyelitis in the US has raised the problem of vaccine-associated disease when oral vaccine is used. Since oral poliovirus vaccine appears to have provided protection through herd immunity, any change in immunization practices that might increase the exposure of susceptibles should be undertaken with great caution. Evidence of repeated importations of wild poliovirus emphasizes this concern. If inactivated vaccine is seriously considered for primary immunization, it should be followed by reimmunization with oral vaccine.