The results showed that TCM and Tnaive were the two main subtypes of S/M/N/E+CD4+ T cells in all four groups (Fig. 5a, b), with a higher proportion of TEMRA and TEM (P < 0.05) and a lower proportion of TCM (P < 0.05) in I-I-I than I-I-δ, M-M-o and I-I-o, respectively (Fig. 5b); this suggested that different variant breakthrough infections and vaccinations affected differentiation of virus-specific CD4+ T cells differently. Here, CD4 is linked to infection.