Studies on severe COVID-19 cases reported exhaustion of T-cells, high expression levels of programmed cell death protein 1 (PD-1), T-cell immunoglobulin and mucin domain 3 (Tim-3), and NKG2A along with the diminished expression of T-cell activation markers CD107, and interferon gamma (IFN-γ) [41]. This evidence concerns the gene PDCD1 and COVID-19.