ObjectiveTo explore the effects of corticosteroid on peripheral blood T lymphocyte subsets in patients with coronavirus disease 2019 (COVID-19).MethodsThis was a retrospective study and 376 patients were included in the study. The patients were classified into three type: moderate type (118 patients), severe type (215 patients), critical type (43 patients). Six critical patients died. T lymphocyte subsets were analyzed and compared among these patients. In severe patients, T lymphocyte subsets were compared between no corticosteroid therapy patients (178 patients) and patients who were treated with corticosteroid for 3 to 5 days (37 patients).Results(1) In contrast with those in moderate patients, in severe patients total lymphocytes [(1359.2±597.9)×106 vs. (1703.7±702.4)×106/L, LSD-t=4.786, P<0.001], total T lymphocytes [(949.2±454.0)×106 vs. (1235.5±555.7)×106/L, LSD-t=5.175, P<0.001] and CD8+ T cells [(336.8±189.8)×106 vs. (461.7±242.8)×106/L, LSD-t=5.332, P<0.001] decreased significantly, and CD4+/CD8+ ratio (1.81±0.92 vs. 1.64±0.74, LSD-t=1.574, P=0.116) was increased. In contrast with those in severe patients, in critical patients CD4+/CD8+ ratio (2.23±1.24 vs. 1.81±0.92, LSD-t=2.627, P=0.009) increased and CD8+ T cells [(232.5±159.8)×106/L vs. (336.8±189.8)×106/L, LSD-t=2.867, P=0.004] decreased significantly, total lymphocytes [(1161.1±583.7)×106/L vs. (1359.2±597.9)×106/L, LSD-t=1.772, P=0.077], total T lymphocytes [(790.5±419.3)×106/L vs. (949.2±454.0)×106/L, LSD-t=1.846, P=0.066] also decreased but without significant difference. There was no significant difference between dead and survived critical patients. (2) In severe type, in contrast with no corticosteroid therapy patients, 37 patients were therapy with corticosteroid for 3 to 5 days, and their total T lymphocytes [(770.6±480.3)×106 vs. (986.3±440.7)×106/L, t=2.666, P=0.008] and CD4+/CD8+ ratio (1.30±0.73 vs. 1.91±0.92, t=3.771, P<0.001) were decreased significantly.ConclusionsIn COVID-19 patients, lymphocytes, T lymphocytes and CD8+ T cells are decreased, but CD4+/CD8+ ratio is increased, and these changes are positively related to the severity of the disease. After corticosteroid therapy, the increase of CD4+/CD8+ ratio is relieved, but T lymphocytes are decreased further.