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Surveillance of influenza in Switzerland between 1987 and 1995
S Chappuis1, M F Paccaud, W Wunderli
1Swiss National Influenza Centre.
Abstract:
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.
Insights
Influenza surveillance in Switzerland (1987-1995) revealed that influenza A viruses were more common than influenza B. Herald viruses helped predict future strains, with A/H3N2 causing significant mortality in the elderly.
Area of Science:
- Virology
- Epidemiology
- Public Health
Background:
- Influenza surveillance is crucial for understanding viral circulation and guiding public health interventions.
- The period 1987-1995 in Switzerland provided a significant dataset for analyzing influenza trends.
Purpose of the Study:
- To analyze the epidemiological patterns of influenza A and B virus circulation in Switzerland from 1987 to 1995.
- To identify the most severe influenza seasons and associated viral subtypes.
- To investigate age-specific isolation rates and mortality patterns related to influenza.
Main Methods:
- Continuous influenza surveillance over 8 years (1987-1995).
- Analysis of predominant influenza virus types and subtypes.
- Tracking seasonal peak activity and outbreak durations.
- Correlation of viral strains with European isolates.
- Identification of 'herald viruses' for predictive analysis.
- Examination of age-specific isolation rates and mortality data.
Main Results:
- Influenza A viruses predominated in 5 seasons, influenza B in 3 seasons.
- Severe outbreaks occurred in 1988/89 (A/H1N1), 1989/90 (A/H3N2), and 1994/95 (A/H3N2 and B co-circulation).
- Influenza A peaked in Dec-Jan (1987-1993), while Influenza B peaked in Feb-Mar.
- The 1994/95 season saw a simultaneous peak of A/H3N2 and B in late March.
- Swiss variants often lagged European isolates by one year.
- Herald viruses accurately predicted subsequent dominant strains.
- A/H3N2 isolation was higher in 10-19 and 60+ age groups; Influenza B isolation decreased with age.
- Increased mortality was observed in those over 65 during A/H3N2 activity.
Conclusions:
- Influenza A and B viruses exhibit distinct seasonal patterns, though co-circulation can occur.
- The 'herald virus' phenomenon is a valuable tool for influenza forecasting.
- Age-specific susceptibility and mortality vary between influenza A subtypes and influenza B.
- Influenza A/H3N2 poses a significant risk to the elderly population, necessitating targeted public health measures.