For the pairwise comparison between mild and severe COVID-19, 24 studies reported decreased counts for CD4 T-cells in the severe cases compared with mild ones (SMD = −1.39 to −0.11), and only 2 studies reported increased counts for CD4 T-cells (SMD = 0.25 to 0.34) (Figure 3A and Supplementary Table 9.1). Here, CD4 is linked to COVID-19.