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 .
C0032374
poliovirus vaccines
C0032371
poliomyelitis