CD4 and COVID-19: In addition, during the severe and mild stages of SARS-CoV-2 infection, a lower proportion of CD8+ T cells and a higher proportion of CD4+ T cells with high levels of effector molecules were detected in severe patients than in mild patients, according to scRNA-seq analysis based on BALF from three severe and three mild COVID-19 patients (Liao et al., 2020a).