CD4 and COVID-19: In clinical trials, the first vaccines that utilized SARS-CoV-2 S proteins by mRNA lipoparticles (Pfizer BNT162b2; Moderna VRC mRNA) or viral vectored vaccines (CanSino AdV5 COVID-19; Oxford/AstraZeneca ChAdOx) indicated high efficiency for SARS-CoV-2-specific CD4+ and CD8+ T cells.