LTB and infection: Moreover, LTB has also proven adjuvant activity when parenterally administered [40]; therefore, the design of combined parenteral–mucosal immunization scheme could be achieved with the LTB-RBD antigen to induce proper immune responses in terms of potency and compartmentalization (i.e., parenteral priming and mucosal boosting), offering the potential to prevent virus spread at early stages of infection.