Differentiating mpox infection and vaccination using a validated multiplex orthopoxvirus IgG serology assay.

The resurgence of monkeypox virus (MPXV) has increased demand for validated serological assays to assess exposure and immunity. Cross-reactivity among orthopoxviruses, stemming from high sequence conservation, complicates distinguishing antibody responses from natural MPXV infection versus vaccination or other orthopoxvirus exposures. We validated the Meso Scale Discovery (MSD) V-PLEX Orthopoxvirus Panel 1 (IgG) Kit, which quantifies antibody levels to five MPXV antigens and their vaccinia virus (VACV) orthologs, following Good Clinical Laboratory Practice guidelines. We assessed assay performance using serum from 26 individuals with prior mpox, 52 JYNNEOS vaccine recipients, and 179 unexposed controls. The assay reliably detected antibody responses in all exposed cohorts with peak levels observed 2 months post-vaccination. Antibody levels to specific antigens also correlated with Modified Vaccinia Ankara neutralization titer, particularly for MPXV B6R/VACV B5R, MPXV E8L/VACV D8L, and MPXV M1R/VACV L1. Receiver operating characteristic analysis showed that some individual antigens achieved high sensitivity and specificity for exposure detection (area under the curve [AUC] > 0.96 for VACV D8L, MPXV B6R, VACV B5R); however, individual antigens performed poorly in distinguishing infection from vaccination. In contrast, antibody level ratios between some MPXV and VACV orthologs effectively differentiated MPXV infection from vaccinia vaccination with high sensitivity and specificity (e.g., MPXV A35R/VACV A33R ortholog ratio, AUC = 0.97, sensitivity = 0.97, specificity = 0.96). Our findings validate the MSD assay for clinical research and serosurveillance to assess MPXV immunity and support the utility of ortholog pair ratio analysis as a strategy to discriminate vaccinated and infected individuals. IMPORTANCE: Mpox continues to spread around the world, with recent data showing increasing incidence in the United States. While there are multiple Food and Drug Administration (FDA)-authorized real-time PCR tests for diagnostic use, there are no FDA-authorized serological tests and few laboratory-developed serological tests offered. We evaluated the Meso Scale Discovery V-PLEX Orthopoxvirus Panel 1 (IgG) Kit according to Good Clinical Laboratory Practice guidelines and found that the assay reliably detected antibody responses in monkeypox virus (MPXV)- and vaccinia virus (VACV)-exposed cohorts and could distinguish them from unexposed cohorts. Intriguingly, we found that antibody level ratios between certain MPXV and VACV orthologs could distinguish prior mpox infection from vaccinia vaccination. Overall, these data highlight the use of multi-antigen panels in challenging scenarios for serological testing, such as the cross-reactivity presented by orthopoxviruses.