Among participants with HAI titer < 40, the odds ratio of infection for every fold increase in pre-existing H3N2-specific CD4 and CD8 T cell responses was 0.68 (95% CI 0.51, 0.90) and 0.67 (95% CI 0.52, 0.87), corresponding to 32% (95% CI 10%, 49%) and 33% (95% CI 13%, 48%) protection, respectively, adjusted for age and the differences in infection risk among epidemics. Here, CD4 is linked to infection.