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 mg Ψ-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.