CD4 and COVID-19: Optimal COVID-19 vaccine immunity requires the activation of both cellular and humoral responses in regard to (1) activation of CD4 T cells to activate B-cell maturation to produce functional antibodies to neutralize SARS-COV-2 as well as B-memory responses and 2) stimulation of CD8 T cell production to eliminate virus-infected cells and the activation of CD8 T memory cells16.