Among participants with HAI titer < 40, the odds ratio of infection for every fold increase in pre-existing sH1N1-specific CD4 and CD8 T cell responses was 0.73 (95% CI 0.60, 0.90) and 0.78 (95% CI 0.66, 0.92), corresponding to 27% (95% CI 10%, 40%) and 22% (95% CI 8%, 34%) protection, respectively, adjusted for age and the differences in infection risk among epidemics (Fig. 5). Here, CD4 is linked to infection.