ACE2 and COVID-19: The decision to focus our immunization efforts on the RBD-protein-decorated rather than the full-length S-protein-decorated VNPs was based on the analysis of sera from convalescent COVID-19 patients, which showed that antibodies binding to folded but not to unfolded RBD can block the interaction of the virus with its cellular receptor, ACE2, and thus have the potential to prevent infection [51,52].