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. In Maryland, polio immunization is required by law for entry into pre-school programs, and kindergarten through grade twelve. 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.