CD4 and COVID-19: The number of regulatory T cells (CD3+CD4+CD25+CD127low+), naïve regulatory T cells (CD45RA+CD3+CD4+CD25+CD127low+), as well as induced regulatory T cells (CD45RO+CD3+CD4+CD25+CD127low+) are decreased in the circulation of COVID-19 patients as compared to healthy controls (Qin et al., 2020).