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

The global spread of monkeypox virus (MPV) underscores the importance of monitoring vaccine effectiveness and cross-protective immunity against MPV and orthopoxviruses (OPXVs), such as vaccinia virus (VACV) and cowpox virus (CPV). This study evaluated humoral OPXV cross-reactivity in individuals infected with clade IIb MPV or health care workers receiving the Modified Vaccinia Ankara-Bavarian Nordic (MVA-BN) third-generation smallpox vaccine.

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. A complement-dependent neutralization assay using MVA-enhanced green fluorescent protein (eGFP) was performed by exploiting a high-content imaging system.

MPV infection induced OPXV-reactive immunoglobulin (Ig)A and IgM within 11 days (median) after SO, with IgG increasing over time. MPV and VACV neutralizing antibodies (nAbs) were sustained and persisted for 65 days (median) from SO, whereas CPV nAbs waned significantly. MVA-BN vaccinees displayed persistent VACV-reactive IgG through 2 years after vaccination, with titers declining throughout, and cross-reactive neutralizing capacity persisted over 2 years. Exogenous complement enhanced the sensitivity of the MVA-eGFP neutralization assay. Infection and vaccination induced MVA-eGFP nAbs that correlated significantly with OPXV nAbs.

MPV infection and smallpox vaccination might confer OPXV cross-protection. IgA may support early mpox diagnosis, particularly in asymptomatic high-risk individuals. A complement-dependent neutralization assay may improve the assessment of cross-immunity.