Age-specific mortality during the 1918 influenza pandemic: unravelling the mystery of high young adult mortality

Abstract

The worldwide spread of a novel influenza A (H1N1) virus in 2009 showed that influenza remains a significant health threat, even for individuals in the prime of life. This paper focuses on the unusually high young adult mortality observed during the Spanish flu pandemic of 1918. Using historical records from Canada and the U.S., we report a peak of mortality at the exact age of 28 during the pandemic and argue that this increased mortality resulted from an early life exposure to influenza during the previous Russian flu pandemic of 1889-90. We posit that in specific instances, development of immunological memory to an influenza virus strain in early life may lead to a dysregulated immune response to antigenically novel strains encountered in later life, thereby increasing the risk of death. Exposure during critical periods of development could also create holes in the T cell repertoire and impair fetal maturation in general, thereby increasing mortality from infectious diseases later in life. Knowledge of the age-pattern of susceptibility to mortality from influenza could improve crisis management during future influenza pandemics.

Figure 1. Recorded deaths (from all causes) by age in Montreal and Toronto, September and October, 1918.

Figure 2. Death rates (from all causes) by age in Montreal and Toronto during the month of October, 1918.

Figure 3. Distribution of deaths (from all causes) by age as a percentage of all deaths between ages 15 and 44 in all available Canadian and American locations.

^* *Key for Figure 3: Yearly age-specific death counts were available from age 18 to 31 for Philadelphia, Indiana, and Kansas in the special tables that were tabulated for these locations in 1920 (27); Outside this range, death counts were only available for collapsed age-groups (i.e., 15–17, 32–34, 35–39, and 40–44). For these age-groups we divided the number of deaths in the interval by its length and plotted the obtained number at the midpoint value of the interval. Other Canadian locations: Hamilton, Ottawa, London, Welland & Lincoln, Winnipeg, and Vancouver.

Figure 4. Percentages of deaths by age from pandemic-related causes and from all other causes in Ontario and US locations, September to December 1918.

^&&Key for Figure 4: Yearly age-specific deaths counts were available from age 0 to 4 and from 18 to 31 for the US locations in the special tables that were tabulated for these locations in 1920 (27); outside these ranges, death counts were only available for collapsed age-groups (i.e., 5–9, 10–14, 15–17, 32–34, 35–39, 40–44, ..., and 60–64). We divided the number of deaths in the interval by its length and plotted the obtained number at the midpoint value of the interval. Deaths above age 65 were not available by age or age-groups for the U.S. locations. Ontario locations: Toronto, Hamilton, Ottawa, London, and Welland & Lincoln; US locations: Philadelphia, Indiana, and Kansas.