CD38 and COVID-19: As previously reported (14, 16, 44), in convalescent COVID-19 patients (n = 9), although the expression of HLA-DR, CD38, and Ki67 in SARS-CoV-2 CD4+ T cells was significantly reduced compared with acute COVID-19 patients (reflecting viral clearance), cells maintained an elevated PD-1 expression and retained their early differentiated phenotype (Supplemental Figure 2C).