CD38 and COVID-19: There was increasing evidence that circulating CD8+ T cells from severe COVID-19 patients with exhibited an exhausted phenotype characterized by increased expression of PD-1, TIM-3, LAG-3, CTLA-4, NKG2A, and CD39, and there was an inverse relationship between the expression of PD-1 and CD38 (an activation marker) in CD8+ T cells (53, 93–96).