Determinants of mortality in naval units during the 1918-19 influenza pandemic.

In 1918, two waves of epidemic influenza arose with very different clinical phenotypes. During the first wave, infection rates were high but mortality was low. During the second wave, high numbers of deaths occurred and mortality differed 30-100 times among seemingly similar groups of affected adults, but the reason for this variation is unclear. In 1918, the crews of most warships and some island populations were affected by influenza during both waves of infection and had no or very few deaths during the second wave. However, some warships and island populations were not affected during the first wave of infection and had high mortality during the second wave. These findings suggest that infection during the first wave protected against death, but not infection, during the second wave. If so, the two waves of infection were probably caused by antigenically distinct influenza viruses--not by one virus that suddenly increased in pathogenicity between the first and second waves. These findings are relevant to modern concerns that the 2009 influenza A H1N1 virus could suddenly increase in lethality.