Broadening SARS-CoV-2 Immunity by Combining ORFV and Protein-Based Vaccines.

BACKGROUND: Emerging immune-evasive viral variants threaten the efficacy of current vaccines, underscoring the need for strategies that elicit broad and durable protection. Heterologous prime-boost regimens combining distinct vaccine platforms can enhance humoral and cellular immunity through complementary mechanisms. METHODS: Using an intramuscular immunization scheme aligned with clinical vaccination practice, CD-1 mice received homologous or heterologous prime-boost regimens combining a replication-deficient Orf virus (Parapoxvirus orf, ORFV)-based spike vaccine (ORFV-S) with the licensed adjuvanted recombinant protein vaccine VidPrevtyn Beta. Spike-specific humoral and cellular immune responses were assessed. RESULTS: ORFV-S alone induced potent and broad spike-specific IgG responses and achieved the strongest ACE2-binding inhibition across variants of concern. ORFV-S priming followed by VidPrevtyn Beta boosting markedly enhanced the magnitude and cross-variant breadth of antibody responses compared with homologous protein vaccination. Both homologous ORFV-S and heterologous regimens incorporating ORFV-S elicited strong CD4(+) and CD8(+) T-cell responses, whereas VidPrevtyn Beta alone induced only modest T-cell activity, demonstrating that ORFV-S effectively complements protein-based vaccines. CONCLUSIONS: The ORFV-S vector represents a potent vaccine platform capable of inducing broad humoral and cellular immunity. Its use in heterologous prime-boost combinations enhances both antibody magnitude and breadth beyond homologous protein vaccination, supporting its application in vaccination strategies against evolving viral pathogens.