CD4 and COVID-19: The immune responses stimulated by the T4 based COVID-19 vaccine were broad and included the following: Th1 and Th2 derived IgG and IgA antibodies in sera, virus-neutralizing antibodies, CD4+ helper and effector T cells and CD8+ killer T cells, Th1-biased cytokines, and mucosal IgG and sIgA antibodies in BALF.