For children, based on the pH1N1 pandemic and subsequently two epidemics, we estimated that the odds ratio of pH1N1 infection for every fold increase in pre-existing sH1N1-specific CD8 T cell responses was 0.78 (95% CI 0.63, 0.97), corresponding to 22% (95% CI 3%, 37%) protection, adjusted for pre-epidemic HAI titer and the differences in infection risk among epidemics. The gene discussed is CD8A; the disease is infection.