CD4 and COVID-19: Urra et al. (2020) showed that a low CD8+ T cell level, but not CD4+ T cell level, was related to the severity and clinical progression of COVID-19. Yin et al. (2021) retrospectively analyzed the clinical data of 48 COVID-19 subjects and found that CD8+ T cells in severe or critical case patients were lower than those in asymptomatic and mild symptom patients. Mallajosyula et al. (2021) reported that CD8+ T cells are much more abundant in COVID-19 patients with mild disease than in those with a more severe illness.