CD4 and COVID-19: ChAdOx1 nCoV-19 (AZD1222, viral vector vaccine), NVX-CoV2373 (protein subunit vaccine), mRNA-1273(mRNA vaccine), BNT162 (including BNT162b1 and BNT162b2, mRNA vaccine), and other COVID-19-candidate vaccines were reported to induce Th1 cell responses.19,26–28 After recognition of the AP-MHC class II complex and T-cell receptor (TCR), CD4+ T cells distributed in peripheral lymphoid organs can differentiate into Th1 cells, which secrete various cytokines, such as interleukin 2 (IL-2), and simultaneously upregulate the expression of related receptors (IL-2R).