Laboratory studies of the 1984 influenza epidemic on the Witwatersrand

A particularly severe outbreak of influenza occurred on the Witwatersrand from May to August 1984, caused sequentially by influenza A (H3N2), B/influenza and influenza A (H1N1) viruses. Although the precise extent of the infection was impossible to determine, valuable anecdotal information was provided by a network of sentinel sampling stations in private practices, clinics and hospitals, representing a cross-section of population groups on the Witwatersrand. This active surveillance programme was invaluable in providing some 85% of all the specimens, the remainder being routine clinical specimens; in addition, isolation was approximately twice as efficient for the actively acquired specimens than for the routine ones. The epidemic affected all individuals approximately equally, regardless of age, race or socio-economic status. Infection with H1N1 virus tended to predominate in the younger age group, 78% of isolates being from subjects under 30 years of age, whereas 71% of H3N2 isolates came from subjects over 30 years of age. The B/influenza isolates tended to be more evenly dispersed. Novel strains of B/influenza and H1N1 viruses were introduced into the country and possibly contributed to the greater than usual severity of the epidemic. An active surveillance programme is essential to monitor the extent of influenza virus activity and to alert virologists to the introduction of new strains, although at present forecasting of future influenza epidemics is not possible with any significant degree of reliability.