programmed cell death protein-1 and T cell immunoglobulin domain and mucin domain-3 levels on CD8+ T cells are increased in overtly symptomatic stages compared with the prodromal stage, and peak levels are detected in severe conditions.26 Moreover, killer cell lectin-like receptor subfamily C member 1 receptor expression on cytotoxic lymphocytes, including NK and CD8+ T cells, is elevated.34,35 Thus, elevated exhaustion levels and reduced functional diversity of T cells may predict severe progression in patients with COVID-19.36 This evidence concerns the gene CD8A and COVID-19.