CD8A and infection: Based on the three H3N2 epidemics in our study period (Fig. 5), the odds ratio of infection for every fold increase in pre-existing H3N2-specific CD4 and CD8 T cell responses was 0.72 (95% CI 0.56, 0.92) and 0.74 (95% CI 0.59, 0.92), corresponding to 28% (95% CI 8%, 44%) and 26% (95% CI 8%, 41%) protection, respectively, adjusted for age, pre-epidemic HAI titer, and the differences in infection risk among epidemics.