CD4 and infection: Similarly, the frequencies of specific central memory CD4+ T-cells and the proportions of central memory T-cells as a percentage of specific CD4+ T-cells were comparable after vaccination and infection (Figure 2B,C) (mean 0.045% ± 0.013 versus 0.033% ± 0.011 and mean 16% ± 2 versus 23% ± 6, respectively).