[Estimation of the range of excess death associated with influenza epidemics: application of a model using annual mortality rates and the seasonal index for determining minimum excess mortality estimates].
OBJECTIVE
To determine the range of excess death associated with influenza in Japan.


METHODS
The monthly rates for mortality from all causes other than accidents (ICD9: E800-E949, ICD-10: V01-X59) were determined from the Japanese national vital statistics for 1987-2003. By employing a model using the annual mortality rates and the seasonal index, the number of expected deaths and the 95% range of mortality in the absence of influenza-associated deaths (normal range) were obtained. Point estimation of excess mortality during an influenza season was performed by calculation of differences between the number of deaths observed and the number of deaths expected. The range of excess death was defined: the lower limit of the excess death was performed by calculation of difference between the number of deaths observed and the upper limit of the normal range. The upper limit of the excess death, on the contrary, was performed by calculation of difference between the number of deaths observed and the lower limit of the normal range. Based on the results of a survey of tendencies in the occurrence of infectious diseases, months showing a high prevalence of influenza and associated deaths were regarded as "months when the rate of death from influenza was 0.9 deaths/100,000 person-years or higher".


RESULTS
The excess death determined by point estimation was largest in 1999, followed by 1995, 1993, 1997, 2000, and 2003. The point estimation (range of excess death) in 1999 was approximately 49,000 (approximate range, 37,000-60,000). Correspondingly, the excess death in 1995 was approximately 38,000 (approximate range, 27,000-48,000). While values in 1993, 1997, 2000, and 2003 were in the approximate range of 21,000 to 25,000, the range of excess death of each year showed the differences: approximate range from 15,000 to 36,000 in 1993, from 18,000 to 31,000 in 1997, from 18,000 to 28,000 in 2000, and from 11,000 to 34,000 in 2003, respectively. From comparison of the range of excess deaths by year, that in 1995 appeared to be the largest among the examined years.


CONCLUSIONS
By considering the range of variation in the number of deaths in the years when no influenza epidemics occurred, the increase (the largest number and smallest number of deaths) associated with elevation the prevalence of influenza could be determined. Estimation of the range of excess deaths can be considered a useful method for understanding the influence of influenza on public health.
Takahashi
Mihoko
M
Division of Human and Science, Faculty of Nursing, Yamanashi Prefectural University.
D004196:Disease Outbreaks; D004812:Epidemiologic Methods; D006801:Humans; D007251:Influenza, Human / Q000401:mortality*; D007564:Japan / Q000453:epidemiology; D015993:Life Tables; D015233:Models, Statistical; D012621:Seasons; D015996:Survival Rate