CD4 and infection: We estimated these protections for children and for adults (Fig. 6) and found that the odds ratio of infection for every fold increase in pre-existing H3N2-specific CD4 and CD8 T cell responses was 0.59 (95% CI 0.43, 0.80) and 0.63 (95% CI 0.49, 0.81), corresponding to 41% (95% CI 20%, 57%) and 37% (95% CI 19%, 51%) protection, respectively, for adults, adjusted for pre-epidemic HAI titer and the differences in infection risk among epidemics.