By monitoring distinct subsets of lymphocytes over time, we found that lymphocytopenia occurred early and persisted in severe/critical COVID-19 (Fig. 3a; Supplementary Fig. S1 and Table S2), and there was a slight increase of T-cell counts (including CD4+ and CD8+ T cells) in severe/critical cases at 10–15 dpi, followed by a decrease between 15 and 22 dpi and a second minor increase thereafter. This evidence concerns the gene CD4 and COVID-19.