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.