Vaccine-associated poliomyelitis in the United States, 1961-1972.

Although poliovirus vaccines have led to a dramatic reduction in the incidence of poliomyelitis in the United States, there is evidence that vaccine-related cases have occurred in both vaccine recipients (recipient cases) and their contacts (contact cases) and that the incidence and epidemiologic characteristics of these cases in the 12-year period 1961-1972 have changed. Before 1965, there were 63 recipient cases compared with only 16 between 1965 and 1972. These latter cases were in younger persons and were more frequently associated with trivalent oral poliovirus vaccine (TOPV) than the former cases. Both groups were predominantly male and had intervals between immunization and onset of illness that clustered between 7 and 21 days. Only three contact cases were recognized before 1965, compared with 39 cases in 1965-1972. The increase was in both children and adults. Unlike recipient cases, contact cases were almost as frequent in females as in males. Evidence that the contact cases were vaccine-related included the apparent clustering between 20 and 29 days of the intervals from vaccine administration to onset of illness and the significantly increased frequency of longer intervals for contact cases in non-household members compared with contact cases in household members. The decrease in recipient cases and the increase in contact cases in 1965-1972 compared with the previous 3-year period are explained by the general curtailing of routine vaccination of adults after 1964, a switch from monovalent oral polio vaccine to TOPV, improved recognition of contact cases and a shift in emphasis from mass vaccination campaigns and communitywide programs to routine vaccination of infants.