Critically ill or severe COVID-19 patients are characterized as lymphocyte activation identified as increased levels of CD38, CD69, and CD44 T cell activation markers and exhaustion of T cells identified as the increased expression of T cell immunoglobulin domain and mucin domain-3 (TIM3), programmed cell death protein-1 (PD1), and killer cell lectin-like receptor subfamily C member 1 (NKG2A) (20). Here, PDCD1 is linked to COVID-19.