CD4 and infection: Further, we noticed that more multifunctionality in virus-specific CD4+ T cells than CD8+ T cells in different breakthrough infections may be due to the immune imprinting from vaccination history, in which I-I could only induce more memory virus-specific CD4+ T-cell response rather than memory virus-specific CD8+ T-cell response (Supplementary Fig. S2a, b); thus recalled virus-specific CD4+ T cells resulted in more multifunctionality than those primary CD8+ T cells upon breakthrough infection.