Regarding regulatory T (Treg) cell populations, significantly lower levels were observed in the case of critical COVID-19 (p = 0.001), both in terms of naïve (CD45RA+CD3+CD4+CD25+CD127low+) and induced (CD45RO+CD3+CD4+CD25+CD127low+) subsets (p < 0.001 and p = 0.007, respectively). This evidence concerns the gene CD4 and COVID-19.