BENEFITS AND LIMITATIONS OF THE WITWATERSRAND INFLUENZA AND ACUTE RESPIRATORY-INFECTIONS SURVEILLANCE PROGRAM

Objective: To establish an ongoing active surveillance programme for acute respiratory infections in general, and influenza in particular. Design: A network of 16 sentinel primary health care providers furnished morbidity information and clinical specimens for virus characterisation supplemented by school absenteeism and regional mortality data. Setting: General practices, hospital outpatient departments and staff clinics in the Witwatersrand area. Participants: Subjects treated for acute respiratory infections by 7 general practitioners, 1 specialist pulmonologist, 4 paediatric outpatient departments, 1 mine hospital and university; factory and institutional staff clinics. Absenteeism data were obtained from 8 primary and 6 high schools in the region (representing 9 000 pupils). Outcome measures: Morbidity information and strain characterisation of influenza isolates as well as other viral respiratory pathogens, school absenteeism, seasonal excess mortality. Results: The most sensitive indicator of influenza activity was virus isolation, which gives an earlier warning signal of an impending epidemic than morbidity or absenteeism parameters. Both morbidity and school absenteeism provided quantitative indicators of the severity of the epidemic. Mortality from all causes showed characteristic winter increases in the 65-year-old and older population which were not seen in younger individuals. Circulating influenza viral strains matched the strains recommended for the vaccine in 1991 and 1992, but not in 1993. Conclusions: The course and extent of the annual winter influenza epidemic can be charted by means of an active surveillance programme, with sentinel primary health care providers furnishing morbidity data arid clinical material from which virus isolations can be made. Antigenic characterisation of the isolates demonstrated that circulating strains may not match recommended strains in northern hemisphere-formulated vaccines and stresses the need for a southern hemisphere vaccine formulation for South Africa. Absenteeism information provides an indicator of the impact of influenza on the economy and excess mortality data emphasise the need for routine immunisation of the elderly.