CCR7 and COVID-19: To determine the involvement of T cells in the pathogenesis of COVID-19, circulating CD3+ total T cells, CD3+CD4+ and CD3+CD8+ T cells, CD3+CD45RA+, CD3+CD45RO+, CD3+HLA-DR, CD3+CD4+CD25+CD127−, CD3+CD4+CD25++ cells as well as CD45RA+CCR7+ (naive), CD45RA+CCR7− (exhausted), CD45RA−CCR7− (effector memory), CD45RA−CCR7+ (central memory) cells in CD3+CD4+ and CD3+CD8+ T cells were analyzed by flow cytometry (Figure 3A).