CD4 and COVID-19: Hou et al. identified a significant enhancement of the expression of inhibitory receptors, which included CTLA-4 on SARS-CoV-2–specific CD4 + T cells (suggesting an exhausted phenotype) even though the quantity of SARS-CoV-2–specific CD4 + T cells in convalescent COVID-19 patients was maintained after a year of recovery [52].