CD4 and COVID-19: However, many unknowns remain, such as whether rapid induction of adaptive immune responses to SARS-CoV-2 and less severe COVID-19 manifestations are driven by pre-existing T cells (114), and whether a role for vaccine-elicited memory CD4+ T cell and CD8+ T cell responses in directly mediating protection from SARS-CoV-2 infection is unclear (115).