S. Chappuis et al., SURVEILLANCE OF INFLUENZA IN SWITZERLAND BETWEEN 1987 AND 1995, Schweizerische medizinische Wochenschrift, 126(26), 1996, pp. 1135-1142
During 8 years of continuous influenza surveillance in Switzerland (19
87 to 1995), influenza A viruses predominated during 5 seasons and inf
luenza B viruses during 3. The most severe outbreaks occurred in the 1
988/89 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 ob
served during December and January (for 8 weeks on average) while infl
uenza B viruses were most active between February and March (ordinaril
y for 6 weeks). During the 1994/95 season, however, simultaneously inc
reased activity of both influenza A/H3N2 and B viruses was observed, p
eaking at the end of March and lasting for about 9 weeks. The variants
predominant in Switzerland largely corresponded antigenically to thos
e isolated elsewhere in Europe, but a delay of one year was often obse
rved. New strains detected at the end of a season (herald viruses) mad
e it possible to forecast the type or subtype of virus which would pre
dominate the next season. Isolation rates for A/H3N2 viruses were sign
ificantly higher in the 10-19 years and 60-plus age groups, whereas th
ere was no statistical difference between age groups for A/H1N1 viruse
s. A decreasing isolation rate, corresponding to increasing age, was f
ound for influenza B viruses. Increased mortality was observed in pati
ents over 65 during the period of high influenza A/H3N2 activity, as w
as especially evident during the 1989/90 season.