Epidemiological Evidence for Viral Exposure in Childhood as a Risk-Factor in Subsequent Influenza Pandemics

James E Oeppen, Max Planck Institute for Demographic Research
Chris Wilson, International Institute for Applied Systems Analysis (IIASA)

The influenza virus offers a simple model for examining Age-Period-Cohort factors in morbidity and mortality. Immuno-senescence is associated with age; the virus changes genetically on a “period” basis; and the acquired immune response is a function of cohort exposure, where the first exposure to influenza in childhood is particularly important. Laboratory methods can be used to analyze immuno-competence by cohort for small samples, but we demonstrate that mortality data can be used to assess the immuno-competence of population cohorts. We apply the technique to epidemics in the last 200 years and show that the impact of genetically new viruses on unexposed-cohorts represents an age-independent proportional increase in the mortality hazard that has declined from 20% to 5%. However, 1918 remains an exception in its scale, non-proportionality, and interaction with previous influenza experience. The results show a clear indication of the importance of childhood exposure and are relevant for pandemic planning.