CD4 and infection: We demonstrated that delivery of these multi-antigen PanCoVax vaccines boosts durable frontline mucosal immunity by enhancing: (1) the frequency and function of tissue-resident cross-reactive CD4+ and CD8+ TRM cells in the lungs; and (2) cross-reactive IgA/IgG antibodies levels in alveolar lavage that limited viral replication to abort infection in the earliest stages and block transmission [79,80,81].