Fig. 2.

Vaccine-induced Th1 cell response. Some COVID-19 vaccines would induce Th1 cell responses. 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). Through IL-2 and IL-2R, T-cell proliferation and CD8⁺ T-cell activation are promoted, CD8⁺ T-cell can differentiate into cytotoxic T lymphocytes (CTLs) through the activation, producing perforin and other cytokines, which may improve the efficacy of vaccines