CD4 and COVID-19: Overall, COVID-19 severity (PC1 severity) strongly correlated with the traits of SARS-CoV-2–specific CD4+ T cells (PC2 phenotype) (P = 0.0006, r = –0.43, Figure 5C), with severe disease being characterized by poor polyfunctional potential, reduced proliferation capacity, and enhanced HLA-DR expression on SARS-CoV-2–specific CD4+ T cells.