CD8A and infection: In patients who had received four vaccine doses, CD4 T cell frequencies were 0.068% (0.034–0.162) before and 0.110% (0.080–0.177) after infection, and CD8 T cell frequencies were 0.039% (0.010–0.145) vs. 0.054% (0.025–0.166) (Fig. 4c and e).