IMPACT OF INFLUENZA ON MORTALITY IN RELATION TO AGE AND UNDERLYING DISEASE, 1967-1989

Based on data from the Dutch Central Bureau of Statistics, the impact of influenza on mortality in The Netherlands was estimated for a 22.5- year period (1967-19891 in four age groups and three entities of disea se, using Poisson regression techniques. Our analysis suggests that, o n average, more than 2000 people died from influenza in The Netherland s each year, but in only a fraction of these deaths was influenza reco gnized as the cause of death. For each case of death registered as cau sed by influenza (registered influenza mortality), 2.6 additional case s of death registered as due to causes other than influenza, neverthel ess, were influenza-related (non-registered influenza mortality). Ther efore, the overall impact of influenza on mortality is estimated to be greater than registered influenza mortality by a factor of 3.6. Those under 60 years of age accounted for 5% of all non-registered influenz a deaths, whereas people aged 60-69, 70-79 years and >80 years account ed for 12%, 29% and 54% of such deaths, respectively. When extrapolati ng the figures for the Dutch population of 1989, we could attribute, o n average per season-year, 82 deaths per 100000 people >60 years, 143 in people >70 years, and 280 in people >80 years. Of all non-registere d influenza cases of death, 47% were estimated to occur in people with heart disease as a primarily reported cause of death, 23% in those wi th lung disease, and 30% in those with other diseases. This study stre sses the serious effects of influenza, mainly in the elderly (95% of n on-registered influenza mortality). Prophylactic measures (vaccination ) should not only be stimulated among populations with defined underly ing diseases, but generally among all people greater-than-or-equal-to 60 years. Future research should address the pathological mechanisms w hich lead to influenza-related excess mortality.