Complement-dependent mpox virus-neutralizing antibodies in infected and vaccinated individuals

ABSTRACT Mpox virus (MPXV) caused a multi-country outbreak in non-endemic areas in 2022. The Modified Vaccinia Ankara (MVA)-based vaccine was used as prophylaxis, but its effectiveness remains poorly characterized. Here, we developed two assays for quantification of neutralizing antibodies (NAbs), using MVA-GFP or a recently isolated MPXV. We measured NAb levels in 470 sera from control, MPXV-infected or MVA-vaccinated individuals. Various levels of MVA NAbs were detected after infection, historic smallpox or MVA vaccination. MPXV was barely sensitive to neutralization. Addition of complement enhanced detection of responsive individuals and NAb levels. Anti-MVA and -MPXV NAbs were observed in 94% and 82% of infected individuals, respectively, and 92% and 56% of MVA vaccinees, respectively. NAb titers were higher in individuals born before 1980, highlighting the impact of historic smallpox vaccination on humoral immunity. Altogether, our results indicate that MPXV neutralization is complement-dependent and help uncover the mechanisms underlying vaccine effectiveness. SUMMARY In 2022, mpox virus (MPXV) caused an unprecedented pandemic outbreak in non-endemic areas. The efficacy of currently available third generation MVA-based vaccines and the nature of the humoral response generated after MPXV infection remain poorly characterized. We established cell-based assays to measure neutralizing antibodies (NAbs) targeting MVA or MPXV. We analyzed 470 sera and detected robust levels of MVA NAbs after infection, historic smallpox vaccination or administration of MVA-based vaccines. Efficient MPXV neutralization required addition of complement. High NAb titers were measured in ancient smallpox-vaccinated MPXV-infected patients, suggesting a potential cross-protection mediated by hybrid immunity.