ABSTRACT: Although mRNA-based COVID-19 vaccines have demonstrated high efficacy, their widespread global use remains constrained by high production costs and cold-chain requirements. Modified vaccinia virus Ankara (MVA) is a highly attenuated and thermostable viral vector with low production costs, potent immunogenicity and strong potential for global distribution. Here, we compared head-to-head the long-term immunogenicity and protective efficacy of an MVA-based vaccine candidate with an approved mRNA vaccine in K18-hACE2 mice, both expressing the SARS-CoV-2 Omicron XBB.1.5 spike (S) protein. Mice received by intramuscular route homologous (mRNA/mRNA and MVA/MVA), heterologous (mRNA/MVA), or single-dose MVA regimens. SARS-CoV-2-specific humoral and cellular responses were evaluated at 10 days and 9 months post-boost, as well as antibody levels at intermediate time points, and protection was assessed following intranasal SARS-CoV-2 XBB.1.5 challenge at 9 months post-vaccination. Binding IgG antibodies against the XBB.1.5 S protein remained high throughout the 9-month period in all vaccinated groups, whereas neutralizing antibody titers peaked early after boosting and progressively declined, converging across regimens over time. S-specific CD8⁺ T-cell responses were strongest in mRNA-containing regimens at day 10 post-boost and, although contracted over time, remain detectable at 9 months post-boost in all two-doses groups, with a trend toward enhanced persistence in the heterologous mRNA/MVA regimen. In contrast, S-specific CD4⁺ T-cell responses remained low across all groups. All two-dose regimens conferred robust protection against SARS-CoV-2 XBB.1.5 challenge, significantly reducing viral RNA levels and infectious titers in both the upper and lower respiratory tract. Transcriptomic analysis of lung tissue after virus challenge revealed that vaccinated animals exhibited reduced expression of genes associated with inflammatory myeloid responses, interferon signalling, and cellular stress compared with infected controls, consistent with attenuated pulmonary inflammation. Notably, mRNA and MVA platforms induced distinct yet convergent protective transcriptional signatures, characterized by a shared reduction of interferon‑driven inflammation alongside differential engagement of humoral versus innate/cellular effector pathways, characteristic of effective antiviral immunity. Overall, our findings demonstrate that MVA‑based vaccination induce durable and protective immunity comparable to mRNA vaccines and support its use as both an alternative and complementary platform to broaden and diversify vaccine‑induced immune responses against SARS-CoV-2 and other emerging respiratory viruses.
