A moderate negative correlation between CD4+ T-cell activation and CD4 counts was observed pre- and post-infection, respectively (r = −0.4553, p = 0.0041; r = −0.3367, p = 0.0388; Figure 4C), which was confirmed as significant in the BBIBP-CorV booster group (r = −0.6400, p = 0.0006; r = −0.3955, p = 0.0503; Figure 4D) but only marginal in the heterologous ZF2001 group (r = −0.0441, p = 0.8880; r = −0.2253, p = 0.4591; Figure 4E). Here, CD4 is linked to infection.