Does exposure to influenza very early in life affect mortality risk during a subsequent outbreak? The 1890 and 1918 pandemics in Canada

Using Canadian data, we explore how exposure to influenza very early in life during the pandemic of 1890 may have influenced mortality risk in the subsequent pandemic of 1918, twenty eight years later. As mortality in 1918 peaked at age 28 in Toronto and in other Canadian cities, we posit that infection with influenza in critical periods of development can result in physiological or immunological impairments that increase risk of death from influenza later in life. The peak at age 28 was most evident in large Canadian cities, while the pattern was still present in a less extreme form in rural areas. The 1918 influenza pandemic occurred during the health transition and, through enduring links to the 1890 pandemic, shows that experiences before the transition may have directly influenced the course of the most severe pandemic of this time period. This study provides new empirical insights connecting early physiological insults and immunological experiences to later life mortality.