Cross-reactive immunity against orthopoxviruses after monkeypox virus infection or smallpox vaccination.

OBJECTIVES: Global spread of monkeypox virus (MPV) underscored the importance to monitor vaccine effectiveness and cross-protective immunity against MPV and orthopoxviruses (OPXVs), as vaccinia virus (VACV) and cowpox virus (CPV). This study evaluated humoral OPXV cross-reactivity in clade IIb MPV-infected individuals or healthcare workers receiving MVA-BN third-generation smallpox vaccine. METHODS: Participants were followed from mpox symptom onset (SO) or MVA-BN vaccination. Binding and neutralizing antibodies were measured by Indirect Immunofluorescence and Plaque Reduction Neutralization assays. Complement-dependent neutralization assay using MVA-eGFP was performed exploiting a high-content imaging system. RESULTS: MPV infection induced OPXV-reactive IgA and IgM within 11 days (median) after SO, with IgG increasing over time. MPV and VACV neutralizing antibodies (nAbs) were sustained and persisted to 65 days (median) from SO, while CPV nAbs significantly waned. MVA-BN vaccinees displayed persistent VACV-reactive IgG through two years after vaccination, with titers declining throughout, and cross-reactive neutralizing capacity over two years. Exogenous complement enhanced sensitivity of MVA-eGFP neutralization assay. Infection and vaccination induced MVA-eGFP nAbs significantly correlating with OPXV nAbs. CONCLUSIONS: MPV infection and smallpox vaccination might confer OPXV cross-protection. IgA may support early mpox diagnosis, particularly in asymptomatic high-risk individuals. Complement-dependent neutralization assay may improve assessment of cross-immunity.