The assessment of the CD8+ T cell maturation and differentiation stages revealed a significantly higher increase in the relative number of EM3 and decreased EM1 cytotoxic T cells in acute COVID-19 patients compared to COVID-19 convalescents (46.18% (25.76; 61.25) vs. 38.16% (18.35; 52.37), p = 0.013 and 33.53% (23.65; 49.34) vs. 42.43% (25.76; 61.25) with p = 0.008, respectively) and healthy individuals (46.18% (25.76; 61.25) vs. 28.67% (10.74; 39.05), p < 0.001 and 33.53% (23.65; 49.34) vs. 52.10% (42.05; 66.44) with p < 0.001, respectively). This evidence concerns the gene CD8A and COVID-19.