These antigen-engineering strategies may also be applicable to measles-based vaccines for other emerging beta coronaviruses.197 Unfortunately, immunogenicity was insufficient after a single IM dose of MeV-based COVID-19 vaccine expressing a pre-fusion stabilized SARS-CoV-2 S protein (V591) in Phase I/II clinical trials, especially in measles-immunized individuals.198,199 Currently the relationship between low immunogenicity and anti-vector immunity is not clear. Here, PROS1 is linked to measles.