A well-known feature of the great H1N1 influenza pandemic of a century ago is that the highest mortality rate was amongst young adults. The general explanation has been that they died from an over-reaction of their active immune systems. This explanation has never been very satisfactory because teenagers also have very active immune systems. Recent virological research provides a new perspective, which is important for life and health insurers. There is now strong recent scientific evidence for the principle of antigenic imprinting, where the highest antibody response is against influenza virus strains from childhood. The peak ages of 1918 pandemic mortality correspond to a cohort exposed to the H3N8 1889–1890 Russian influenza pandemic. The vulnerability of an individual depends crucially on his or her exposure to influenza during their lifetime, especially childhood. Date of birth is thus a key indicator of pandemic vulnerability. An analysis of the implications is presented, with focus on those now in their fifties, who were exposed to the H3N2 1968 Hong Kong influenza.
