GZMK and COVID-19: The proportions of GZMK- and GNLY-expressing CD4+ T cell subtypes in P_MD group were significantly lower than that of A_MD group (both p = 0.014) (Figure 7B), suggesting a less-activated T cell response in children. In addition, downregulated cytotoxicity-associated genes (e.g., GZMA, GZMB, GZMH)(Figure 7C), suppressed T cell receptor signaling pathway (Figure 7D), and lower gene set scores of activation, proliferation, and apoptosis GO terms (Figures 7E, F, Supplementary Figure 15), were all suggestive of a less active T cell response in pediatric COVID-19 (P_MD group).