Multiple monovalent COVID-19 PHH-1V vaccine adaptations elicit homologous and cross-neutralizing antibody responses in mice

As SARS-CoV-2 continues to evolve, rapidly adaptable COVID-19 vaccines remain essential. We evaluated monovalent, variant-adapted PHH-1V-based vaccine constructs in BALB/c mice after two-dose primary vaccination and as heterologous boosters in mRNA-primed animals. Adaptations comprised Omicron XBB.1.16, JN.1, XEC, and LP.8.1 receptor-binding domain (RBD) homodimers (2 μg/dose) in squalene-based adjuvant. Immunogenicity on day 14 post-vaccination was assessed by pseudovirus-based neutralization and RBD-specific IgG assays; tolerability was monitored throughout. All adapted boosters significantly elicited variant-matched neutralizing and binding responses relative to baseline. In pairwise comparisons, all adapted boosters except LP.8.1 tended to outperform the preceding construct in homologous neutralization ( p < 0.0001, XBB.1.16-adapted; non-significant for others), while binding IgG remained comparable. In primary vaccination, updated constructs increased matched-variant neutralization versus predecessors; binding IgG was similar overall. Vaccination was well tolerated, limited to transient, mild injection-site inflammation. These data support PHH-1V original vaccine as an updatable platform, ensuring broad, campaign-ready SARS-CoV-2 variant coverage.