Contributing to the sustained interruption of wild-type polio virus circulation in the United States are high immunization levels and the spread of the OPV strains of virus to unimmunized populations. Since the 1980–1981 school year, the levels of immunization with three or more doses of OPV at the time of school entry, about 5 years of age, have been greater than 90 percent. Immunization levels among U.S. preschool-age children, though, are substantially lower. In 1993, the reported coverage rate among 19- to 35-month-old children (median age, 27 months) for three doses of OPV was 79 percent. This translates into approximately 1.25 million 19- to 35-month-old children in 1993 who had not received a full primary series of OPV. The importance of the spread of the vaccine strains of virus in maintaining immunity to polio among unvaccinated inner-city populations was investigated by Chen and colleagues in emergency rooms in Houston and Detroit in 1990–1991. Their unpublished data suggest that spread of the OPV strains plays a role in enhancing population-based immunity among unvaccinated children in inner cities.
The U.S. has several surveillance systems for polio disease: (1) routine reporting through the morbidity reporting system for contagious diseases; (2) the Vaccine Adverse Event Reporting System (a passive surveillance system, stimulated by provider letters, to which vaccine-associated cases of polio may be reported); (3) the enterovirus surveillance system, which collects reports from about 30 states and one county on their isolation of enterovirus; (4) the large linked database system (LLDB), a system in four large health maintenance organizations in the western United States that covers about 500,000 children from birth to age 6 and includes surveillance for paralytic illness; (5) investigation of clusters of cases of paralytic polio; and (6) reports from investigators who isolate the virus and call CDC to request characterization of wild-type poliovirus. Current plans are to continue the system as it is structured and to continue to stimulate reporting through publicity.
Between 1975 and 1992, 189 confirmed cases of paralytic poliomyelitis disease were reported in the United States. These included 10 epidemic cases, 152 vaccine-associated cases, 14 imported cases, and 13 cases of indeterminate origin. Since 1980, no epidemic or indigenously acquired cases of paralytic poliomyelitis caused by wild-type virus have been detected in the United States. During the 5-year period from 1990 through 1994, only one case of poliomyelitis importation was reported, an intentional importation of a 30-month-old who was brought to the United States for treatment following the onset of disease in Nigeria.