CD8A and COVID-19: The clinical characteristics of COVID-19 present serious disturbances in neutrophils, lymphocytes, and CD8+ T cells in peripheral blood [7,53] and melatonin exerts regulatory actions on the immune system and directly enhances the immune response by improving the proliferation and maturation of natural killer cells, T and B lymphocytes, granulocytes, and monocytes in both bone marrow and other tissues [54].