Moreover, our results showed that there was a significant positive association between the IgG antibody titer level and CD4+ and CD8+ T-cell response to COVID-19 epitopes (Ag1), (Ag2), and (Ag3) after the first vaccine prime (T1) and the second vaccination (T2) time points similar to previous studies published on non-cancer individuals infected with SARS-CoV-2 virus [46,47,48]. Here, CD4 is linked to COVID-19.