The excess mortality during the influenza-epidemic periods
(IASR 1999; 20: 293-294)

Excess death, or excess mortality, has been observed, when large epidemics of influenza occurred. Excess mortality results from various deaths due to acute respiratory illnesses, such as influenza, pneumonia, acute bronchitis, as well as cardiac illnesses, cerebrovascular illnesses, and diabetes mellitus. Excess mortality can be observed by increase of the annual death toll.

Death from influenza is often diagnosed as pneumonia. Thus, in order to estimate impact of influenza epidemics, death toll due to influenza and pneumonia is often used (table). According to the National Epidemiological Surveillance of Infectious Diseases, large influenza epidemics, in which more than 30.0 influenza-like illnesses were reported per sentinel, were observed in 1989/90, 1992/93, 1994/95, 1997/98, and 1998/99 seasons. Figure shows that death toll of pneumonia and influenza is above the baseline, and the excess mortality occurred. In 1996/97 season, excess mortality was observed, in spite of small number of influenza patient. The baseline was calculated by autoregressive integrated moving average (ARIMA) method. Excess mortality is defined when the death toll exceeds 95% confidence interval.

Majority of excess death is estimated death of the elderly population, since more than 90% of death due to influenza and pneumonia attribute to the elderly persons aged 65 years and over. The figure shows a fact that the rate of population aged 65years and over is increasing every year. The results will be explained, considering 1) aging leads increase the rate of population with chronic illnesses, 2) such population develop weaker immune response against influenza, and 3) aging itself.

Reported by Kiyosu Taniguchi and Nahoko Shindo, Infectious Disease Surveillance Center, NIID.

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