Mortality risk factors during the 1918-1919 influenza pandemic in the Australian army.

BACKGROUND Understanding the risk of mortality during the 1918-1919 influenza pandemic could inform preparations for a future pandemic. METHODS Prospectively collected demographic, hospitalization, and death data from all individuals who served in the Australian Imperial Force from 1914 through 1919 in Europe and the Middle East were abstracted from archived records. Analyses were conducted to determine mortality risk factors. RESULTS Hospitalization with a respiratory illness during the spring-summer of 1918 protected soldiers from death (odds ratio, 0.37 [95% confidence interval, 0.25-0.53]; P < .001) but not from hospitalization during the fall-winter of 1918-1919. During the fall-winter of 1918-1919, there was a strong inverse relationship between risk of dying of pneumonia-influenza and time in military service. The pneumonia-influenza death rate among men who enlisted in 1918 (6.33 deaths per 100 person-years) was 9 times higher than that among the 1917 enlistment cohort (0.72 deaths per 100 person-years) and >14 times higher than that among the 1916 cohort (0.43 deaths per 100 person-years), 1915 cohort (0.29 deaths per 100 person-years), and 1914 cohort (0.28 deaths per 100 person-years). CONCLUSION There was a strong inverse relationship between length of service in the Australian Imperial Force and mortality risk from pneumonia-influenza during the fall-winter of 1918-1919. The protective effect of increased service likely reflected increased acquired immunity to influenza viruses and endemic bacterial strains that caused secondary pneumonia and most of the deaths during the 1918-1919 influenza pandemic.