CD4 and COVID-19: To obtain a complete picture of the adaptive immune response to anti–COVID-19 vaccination, we evaluated the antigen-specific CD4+ T cell response to SARS-CoV-2 spike protein peptides, monitoring CD154 expression and the production of IL-2, IFN-γ, and TNF-α upon in vitro stimulation (Figure 4A and Supplemental Figure 1).