Five years of sentinel surveillance of acute respiratory infections (1985-1990): the benefits of an influenza early warning system.

For the last five years, the Brussels Institute of Hygiene and Epidemiology has been involved in the surveillance of acute respiratory infections (ARI). The four indicators used (number of encounters of ARI by GP's/100 encounters, virus isolations, absenteeism and mortality) are discussed. A regression procedure is applied to the data collected by a sentinel network of general practitioners (GP's). This procedure permits the baseline to be visualized and an epidemic threshold to be determined in order to recognize early an influenza outbreak. The traditional use of flu-like illnesses as an indicator might be improved by the addition of non-specific ARI which are more precocious, especially in children. The criteria for an accurate definition of an influenza epidemic are discussed. The same mathematical model can be used for the analysis of mortality linked with an outbreak. It shows that the last epidemic in the winter 1989-1990 was responsible for about 4900 deaths directly or indirectly related to influenza.