CD4 and COVID-19: In severe COVID-19 the ratio between naïve CD4+ and CD8+ cells was weakly increased, without achieving a significant difference; on the opposite, the ratio between effector CD4+ and CD8+ cells was progressively reduced from mild through moderate and severe disease (p = 0.01, ANOVA-test), due to the combined reduction of both CD4+ and CD8+ cells in severe disease, as shown in Fig. 3A–I.