Influenza-attributable Mortality Among the Elderly in Switzerland

Background: Influenza infections are considered responsible for a substantial burden of disease and mortality in the elderly, especially during winter time. However, death certificates indicating influenza as the cause of death might only partly reflect the mortality attributable to influenza.

Methods: We estimated influenza-attributable mortality for the Swiss resident population of age 60 and older from 1969 to 1999 by Poisson regression modelling of all cause and influenza mortality, and examined long-term trends by age and gender. In sensitivity analyses we additionally used data on official pneumonia deaths, as well as clinical diagnosis of influenza-associated illnesses from the Swiss Sentinel Network.

Results: For the 30 successive respiratory seasons (July of a given year to June of the next year) from 1969/70 to 1998/99 the estimated total number of influenza-attributable deaths in the Swiss population of 60+ was 24 800 (95% confidence interval: 21 000 to 28 600), about 2 times the official count of influenza deaths. Influenza-attributable mortality rate declined from 1969 to 1999, but the yearly number of influenza-attributable deaths nevertheless stabilised at around 600 to 700 in the ninety's due to aging of the population. The oldest-aged groups persistently showed the highest influenza mortality rate. Influenza-attributable mortality estimates were substantially higher when using the general practice influenza indicator (by 66%) or the combined cause-of-death category pneumonia and influenza (by 169%).

Conclusions: Only counting official influenza deaths underestimated influenza-attributable mortality in Switzerland by a factor of two to three. Despite a gradual decline in age-specific influenza attributable mortality rates in the years 1969 to 1999, we estimated an average annual number of 830 deaths in the elderly Swiss resident population. The elderly remain the primary target group for influenza vaccination to reduce influenza-attributable mortality.