Host and environmental factors reducing mortality during the 1918-1919 influenza pandemic.

Mortality from influenza and pneumonia during the 1918-1919 pandemic was compared between subgroups of civilian and military populations from states in Australia and the USA. Exposures to crowded environments before and during the pandemic were used as proxies for exposure to respiratory infections. In three separate datasets, civilian mortality from influenza and pneumonia was higher in urban than rural populations. In contrast soldiers from these same urban backgrounds had significantly lower mortality than their rural counterparts. This suggests the lower mortality in rural civilians was due to the rural environment, probably due to the relative social isolation in rural areas. This is encouraging for pandemic planning, as it suggests social distancing interventions have the potential to reduce mortality in future pandemics. Soldiers recruited before 1918 had significantly lower mortality than those recruited in 1918, and this effect was separate from the protection given by urban origin to soldiers. Both these effects substantially reduced mortality in soldiers. Further research to identify the mechanisms of these separate protective effects may yield important evidence to inform pandemic planning strategies.