CD4 and infection: For cross-strain protection, 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 and CD8 T cell responses was 0.72 (95% CI 0.59, 0.89) and 0.78 (95% CI 0.67, 0.92), corresponding to 28% (95% CI 11%, 41%) and 22% (95% CI 8%, 33%) protection, respectively, adjusted for age, pre-epidemic HAI titer, and the differences in infection risk among epidemics (Fig. 5).