Pandemic versus Epidemic Influenza Mortality: A Pattern of Changing Age Distribution

Almost all deaths related to current influenza epidemics occur among the elderly. However, mortality was greatest among the young during the 1918–1919 pandemic. This study compared the age distribution of influenza-related deaths in the United States during this century's three influenza A pandemics with that of the following epidemics. Half of influenza-related deaths during the 1968–1969 influenza A (H3N2) pandemic and large proportions of influenza-related deaths during the 1957–1958 influenza A (H2N2) and the 1918–1919 influenza A (H1N1) pandemics occurred among persons < 65 years old. However, this group accounted for decrementally smaller proportions of deaths during the first decade following each pandemic. A model suggested that this mortality pattern may be explained by selective acquisition of protection against fatal illness among younger persons. The large proportion of influenza-related deaths during each pandemic and the following decade among persons < 65 years old should be considered in planning for pandemics.