CD4 and infection: For adults, 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 CD4 T cell responses was 0.66 (95% CI 0.49, 0.87), corresponding to 34% (95% CI 13%, 51%) protection, adjusted for pre-epidemic HAI titer and the differences in infection risk among epidemics.