Observations on mortality during the 1918 influenza pandemic.

The original purpose of our study was to examine the unusual W-shaped mortality curve associated with the 1918 influenza pandemic and possibly explain the peak in mortality among individuals aged 20-40 years. We plotted age-specific excess mortality instead of total mortality for the 1918 pandemic using a 5-year baseline. For comparison, we also graphed excess mortality curves for the 1957 and 1968 pandemics using 5-year baselines. The 1957 and 1968 curves exhibited the usual U-shaped curve, with high excess mortality among infants and the elderly population relative to young adults. The 1918 curve, however, presented unexpected results. A peak in excess mortality among infants and young adults was seen, but the expected W shape did not result. We instead found negative excess mortality among elderly individuals, suggesting that this group was exposed, at an earlier date, to an influenza strain similar to the so-called Spanish influenza (H1N1) strain.