Neutralising antibody responses to MPXV clades Ia, Ib, and IIb after infection or vaccination: a multicountry...

The 2022 global outbreak of monkeypox virus (MPXV) clade IIb prompted widespread use of the modified vaccinia Ankara-Bavarian Nordic (MVA-BN) vaccine which conferred partial protection and contributed to outbreak control. In 2024, the emergence and regional expansion of MPXV clade Ib in central Africa raised new public health concerns and prompted questions about the extent to which immunity elicited by previous clade IIb infection or vaccination confers cross-neutralising immunity against clade Ib. We aimed to assess neutralising and binding antibody responses to MPXV clades Ia, Ib, and IIb after infection or vaccination.

In this multicountry, observational study, plasma samples were collected from three academic medical sites in the USA, Brazil, and Portugal between July 7, 2022, and Aug 3, 2023. Individuals contributing samples were categorised into five cohorts based on their vaccination and infection status: individuals vaccinated with a single dose of the first-generation smallpox vaccine Dryvax, individuals vaccinated with MVA-BN, individuals vaccinated with Dryvax and MVA-BN, individuals with PCR-confirmed MPXV clade IIb infection, and individuals who were neither vaccinated nor had a previous MPXV infection (control cohort). For individuals vaccinated with Dryvax, vaccination occurred during childhood between 1961 and 1987. For individuals in the MVA-BN and MVA-BN plus Dryvax cohorts, MVA-BN was administered at participating sites between July 7, 2022, and Aug 3, 2023. Plasma samples were collected at a single timepoint in the Dryvax, MPXV clade IIb infection, and control cohorts and at multiple predefined timepoints in the MVA-BN and MVA-BN plus Dryvax cohorts. Individuals younger than 18 years and pregnant people were excluded. Individuals in the vaccination cohorts had no history of MPXV infection or exposure. Antibody binding was measured by ELISA with inactivated whole-virus antigens and recombinant proteins; neutralisation activity was assessed by plaque reduction neutralisation tests (PRNT₅₀). Vaccina virus (VACV) was included as a reference control for vaccine-induced immunity. For ELISA and neutralisation assays, thresholds of positivity were defined as the median plus 3 SDs of values obtained from individuals in the control cohort.

All eligible participants with available plasma samples meeting inclusion criteria were included in the study. We analysed 232 plasma samples from 191 participants: 40 samples from 40 individuals in the Dryvax cohort, 62 samples from 33 individuals in the MVA-BN cohort, 31 samples from 19 individuals in the Dryvax plus MVA-BN cohort, 77 samples from 77 individuals in the MPXV clade IIb cohort, and 22 samples from 22 individuals in the control cohort. Across vaccinated cohorts, polyclonal plasma antibodies exhibited cross-reactive binding to MPXV clades Ia, Ib, and IIb, with detectable binding observed in 33-81% of plasma samples, depending on vaccine regimen. Detectable MPXV neutralisation (PRNT₅₀ above threshold) was observed in 28-35% of plasma samples from the Dryvax cohort and 39-55% of plasma samples from the Dryvax plus MVA-BN cohort, whereas responder frequencies were lower among plasma samples from the MVA-BN cohort (3-15%). Median PRNT₅₀ values against MPXV clades Ia, Ib, and IIb ranged from 1·00 to 1·03 across vaccination cohorts, whereas higher neutralisation values were observed against VACV (ranging from 1·48 to 2·22 across cohorts). Individuals in the MPXV clade IIb infection cohort exhibited broad antibody binding but low neutralisation of MPXV clade Ib virus (median log₁₀PRNT₅₀ 1·02 [1·02-1·30] for clade Ia, 1·00 [1·00-1·00] for clade Ib, and 1·20 [IQR 1·00-1·76] for clade IIb). At the antigen level, A35-targeted antibodies bound less efficiently to clade I A35 variants compared with clade IIb.

Humoral immunity elicited by historical or contemporary smallpox vaccination was associated with little neutralising activity across MPXV clades. Of note, previous MPXV clade IIb infection induced cross-reactive antibody binding but low-magnitude neutralising activity against clade Ib. Although additional immune mechanisms could contribute to protection, these findings highlight important constraints in antibody-mediated cross-clade immunity and emphasise the need for vaccines and antibody-based countermeasures that address antigenic divergence among orthopoxviruses.

Yale University, Stavros Niarchos Foundation Institute for Global Infectious Disease, Icahn School of Medicine at Mount Sinai, US National Institutes of Health, and US National Institute of Allergy and Infectious Diseases.