Although parenteral delivery of the CTB-SARS-CoV-2-ACE-2-RBD vaccine generally produced significantly higher levels of anti-ACE-2-RBD-specific antibodies than oral immunization (Figure 7), because the amount of virus passing through the mucosal epithelial cell barrier is often low in comparison with virus replication in the body’s cells, fewer mucosal IgA antibodies may be required to protect against infection by intercepting virus passing through mucosal epidermal cells into the circulation. This evidence concerns the gene ACE2 and infection.