Epidemic of mumps in a partially immune population.

The incidence of mumps in vaccinated and nonvaccinated schoolchildren was studied after a recent epidemic. Information was collected by telephone interviews with the parents and a review of the physicians' records. The vaccine appeared to be effective, for the incidence of mumps in the 145 vaccinated children--5.5%, or 8 cases--was significantly less (P less than 0.001) than the incidence in the 350 children considered susceptible to infection--21.7%, or 76 cases. The percentage of children who had been immunized decreased with increasing age, and acquisition of immunity through natural infection had the reverse trend; thus, the proportions of children susceptible to infection in each age group were about the same, and the age-specific attack rates were similar. Although the mothers were accurate in indicating absence of vaccination, they incorrectly indicated vaccination of their children 43.0% of the time; this error in reporting could influence vaccine administration in older children. Our findings suggest that mumps vaccination may substitute for natural illness in immunizing populations, and that more extensive use of the vaccine over a broader age range is required to prevent similar epidemics in the future.