Poliomyelitis outbreak in South Africa, 1982. II. Laboratory and vaccine aspects.

Between May and September 1982 an extensive epidemic of poliomyelitis occurred in Gazankulu in the north-eastern region of South Africa, with a total of 260 paralysed cases and 42 deaths. A three-part study was instituted, the first of which established the aetiological agent of the epidemic to be poliovirus type 1. This was determined initially by serology and later by isolation of the virus, chiefly from stool specimens. The potency of the trivalent oral polio vaccine manufactured at the National Institute for Virology, was evaluated in the second study by in vitro potency testing of samples recalled from the epidemic areas and also in vivo by determining the serological response of seronegative children immunized with fresh vaccine. Nearly half of the vaccine samples withdrawn from the field had sub-optimal titres whereas fresh vaccine produced a very satisfactory serological response. In order to plan immunization rationally, the third study, namely the determination of the immune status of the various populations throughout the country was instituted. This involved both serology, using a neutralization test on randomly collected sera and also an analysis of patient history and health card documentation. With the exception of the urban black area (Soweto), which had high levels of immunity both serologically (75%) and on history, other areas examined had poorer levels of immunity of approximately 52% on serological testing and 57% on history. It is clear that the epidemic was multifactorial in origin, with fall-offs in the level of immunization and evidence of breaks in the cold chain being the major factors.