Session: Slide Session: Influenza
Sunday, October 26, 2008: 12:00 AM
Room: Room 202B
Background: Improved understanding of the epidemiology of the 1918-19 influenza pandemic might provide useful insights into a future worst-case pandemic scenario. Methods: A case control design was used as well as construction of survival plots for all soldiers based on their year of enlistment. Medical and administrative records of all members of the first Australian Imperial Force (AIF) from 1914-19 (n=340,000) were collected and entered into a database containing prospectively collected demographic and disease information. Results: The risk of death was much higher in the Australian Army for soldiers whose service totalled < 4 months compared to > 1 year (odds ratio = 11.5; CI 5.53, 23.84; p< 0.001). Soldiers enlisting in 1914 had a cumulative pneumonia death rate of 1.7% compared to those enlisting in 1918 whose pneumonia death rate was 3.7%. Since low rank, young age, and short time since enlistment are all closely linked, we examined these related factors to determine if they could be differentiated. Soldiers with rank of Sergeant or above had a lower pneumonia/influenza mortality rate (odds ratio = 0.65; CI 0.45 - 0.94; p < 0.05) compared to privates but this was not significant after adjustment for time in military service. Age was a mortality risk factor consistent with the widely observed peak in mortality in the 20-30 year age group. Conclusions: If the influenza virus causing the 1918-19 pandemic was genuinely new to the human population, then a non-viral explanation must be sought for the marked decrease in mortality risk after a few months military service. These data support the hypothesis that secondary bacterial infections were the predominant cause of death in soldiers during the 1918-19 influenza pandemic and suggest that future pandemic preparations should emphasize the prevention and treatment of secondary bacterial infections.