Enhanced Omicron subvariant cross-neutralization efficacy of a monovalent SARS-CoV-2 BA.4/5 mRNA vaccine encoding a noncleaved, nonfusogenic spike antigen

The rapid emergence of diverse SARS-CoV-2 variants, notably the Omicron variant, poses challenges to vaccine development. Here, we present a noncleaved, nonfusogenic spike (S) protein eliciting robust B- and T-cell immune responses against Omicron BA.5. The antigen incorporates the R685S and R815A mutations, effectively preventing the shedding of the S1 subunit and eliminating fusogenic activity of the resulting S antigen, termed S(SA). Through reverse genetic analysis, we found that the noncleaved form S protein with the R685S mutation enhances ACE2-dependent viral entry in vitro compared to the wild-type S protein, without increasing the virulence of the mutant virus in mice. The mRNA vaccine encoding the Omicron BA.4/5 S(SA) antigen conferred protective immunity in mice following two doses of 1 ug psi-UTP- or UTP-incorporated mRNA vaccines. Despite a roughly 6-fold reduction in neutralizing potency, both mRNA vaccines exhibited broad neutralizing efficacy against Omicron subvariants, including the XBB lineage variants XBB.1.5 and XBB.1.16. ### Competing Interest Statement J.-W.O. and H.Y.S. are inventors of a patent on the mRNA vaccine used in this study, filed by the University Industry Foundation (UIF), Yonsei University. J.W.O holds the positions of founder and CEO at RpexBio Inc., Seoul, Korea. H.G.J is an employee of RpexBio Inc. All other authors declare no competing financial interests.