Serological differentiation between naturally acquired mpox and MVA-BN-vaccine induced antibody responses using ratios of MPXV and VACV antigen pairs in the MSD immunoassay.

This study's aim was to evaluate and benchmark the gold-standard methods neutralization test (NT) and immunofluorescence (IF), against a model linked to a multiplex immunoassay method for analysis of the highly cross-reactive serological responses against the monkeypox (MPXV)- and vaccinia (VACV) viruses. Serum samples from men who have sex with men were analyzed (22 with previous PCR-confirmed mpox episode, 53 MVA-BN-vaccinated, 61 unexposed to infection/vaccination). In-house NTs and IF established titers of mpox- and vaccinia-specific neutralizing and binding IgG antibodies, respectively. The commercial Orthopoxvirus Panel 1 multiplex immunoassay (Meso Scale Discovery [MSD]) was used to measure IgG against five homologous MPXV and VACV antigens. Antibody titers were studied in relation to infection and vaccination. The gold-standard NT and IF assays had ≥90% sensitivity identifying mpox-infected individuals. Despite high specificities in the unexposed group (≥91.8%), serum samples of vaccinated individuals strongly cross-reacted in the gold-standard assays with specificities of 73.6% (MPXV NT) and 43.4% (MPXV IF). Similarly, in the MSD assay, no individual antigen could differentiate between MPXV- or VACV-specific IgG antibodies. However, using ratios of IgG concentrations against three MPXV/VACV antigens enabled classification of separated mpox infected, vaccinated, and unexposed individuals with 88.9% accuracy. Through an innovative analytic approach, utilizing antibody response ratios to homologous MPXV and VACV antigen pairs, we demonstrate a model which differentiates MPXV-infected individuals from vaccinated and unexposed. This approach shows promise as a tool for transmission surveillance and outbreak monitoring, particularly in resource poor settings with low diagnostic capacity, and to prioritize vaccine. Our study provides a uniquely broad perspective on orthopox serology by contrasting the gold standard NT and IF methods with the novel MSD multiplex assay and by providing an analysis model of multiplex data to separate previously mpox-infected, MVA-BN-vaccinated, and unexposed individuals.