CD4 and COVID-19: Also, in critically ill COVID-19 patients, a lower CD8+/CD4+ T-cell ratio was discovered (i.e., a low titer of CD8+ T-cells), suggesting that functional CD8+ T-cells, but better associated with CD4+ T-cells in SARS-CoV-2 infection, are therefore required for preventing infection severity associated with a better viral clearance (24, 25, 28, 29, 53, 88).