Surveillance of influenza in Switzerland between 1987 and 1995.

During 8 years of continuous influenza surveillance in Switzerland (1987 to 1995), influenza A viruses predominated during 5 seasons and influenza B viruses during 3. The most severe outbreaks occurred in the 1988/89 season (A/H1N1 subtype), in the 1989/90 season (A/H3N2 subtype) and in the 1994/95 season (simultaneous outbreak of influenza A/H3N2 and B). From 1987 to 1993, peak activity of influenza A viruses was observed during December and January (for 8 weeks on average) while influenza B viruses were most active between February and March (ordinarily for 6 weeks). During the 1994/95 season, however, simultaneously increased activity of both influenza A/H3N2 and B viruses was observed, peaking at the end of March and lasting for about 9 weeks. The variants predominant in Switzerland largely corresponded antigenically to those isolated elsewhere in Europe, but a delay of one year was often observed. New strains detected at the end of a season (herald viruses) made it possible to forecast the type or subtype of virus which would predominate the next season. Isolation rates for A/H3N2 viruses were significantly higher in the 10-19 years and 60-plus age groups, whereas there was no statistical difference between age groups for A/H1N1 viruses. A decreasing isolation rate, corresponding to increasing age, was found for influenza B viruses. Increased mortality was observed in patients over 65 during the period of high influenza A/H3N2 activity, as was especially evident during the 1989/90 season.