Polio

Current Case Definition for Surveillance

The clinical case definition is: "Acute onset of a flaccid paralysis of one or more limbs with decreased or absent tendon reflexes in the affected limbs, without other apparent cause, and without sensory or cognitive loss." Probable cases must meet the clinical case definition. Confirmed cases must meet the clinical case definition, and have a neurological deficit 60 days after onset of initial symptoms, or die, or have unknown follow-up status. Final classification is made by an expert panel and only confirmed cases are included in the MMWR. Residual paralytic disease occurs in approximately 10% of cases with paralytic illness. Aseptic meningitis occurs in 1% to 5% of patients with non-paralytic minor illness.

Photo Courtesy of the World Health Organization

Immunization

The inactivated polio vaccine (IPV) was licensed in 1955 and the trivalent live oral polio vaccine (OPV) in 1963. Since 1979, IPV has accounted for about 0.1% of all polio vaccine administered every year in the United States. After a primary series of three doses of OPV, seroconversion to all three types is at least 99%. Three properly spaced doses of OPV have been estimated to have a clinical efficacy of virtually 100% in a developed country setting and should confer lifelong immunity.

In Maryland, polio immunization is required by law for entry into pre-school programs, and kindergarten through grade twelve. Based on the 1998/99 retrospective kindergarten survey, an estimated 86% of children in Maryland had received three doses of OPV by 24 months of age (compared to 85% in the 1988/89 survey). An all-IPV schedule (all four doses) is currently being recommended for routine childhood vaccinations in the United States in order to eliminate the risk of vaccine-associated paralytic polio. Children should receive the IPV doses at 2 months of age, 4 months, between ages 6 and 18 months, and between ages 4 and 6 years. 

There was substantial fluctuation in the number of reported cases of paralytic polio in Maryland from the 1912 to 1960, with a maximum of 481 cases reported in 1950. Paralysis is more likely when infection occurs in older individuals. Thus, the appearance of the graph of the five year mean rates may be associated with a decline in incidence through the 1920's related to improvements in sanitation, followed by an increase in the average age of infection. There was a steep decline in the five-year mean incidence of polio in Maryland from the early 1950's through the early 1960's. The last year in which more than two cases were reported in Maryland was 1961, when 35 cases were reported. Fifteen cases have been reported since 1961. All six cases of paralytic polio reported in Maryland since 1977 were vaccine associated.

Figure 1. Reported Cases of Paralytic Polio in Maryland, 1970-1999.

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