Discordant performance of mpox serological assays

Background: Since July 23, 2022, global mpox cases reached 92,546, with over 31,000 in the United States. Asymptomatic carriage is a critical mechanism influencing the global dissemination of mpox. Seroprevalence studies are crucial for determining the epidemic's true burden, but uncertainties persist in serologic assay performance and how smallpox vaccination may influence assay interpretation.

Objectives: Our study aimed to assess the performance of several diagnostic assays among mpox-positive, vaccinated, and pre-outbreak negative control samples. This investigation sought to enhance our understanding and management of future mpox outbreaks.

Study design: Serum samples from 10 mpox-positive, five vaccinated uninfected, and 137 pre-outbreak controls were obtained for serological testing. The mpox-positive samples were obtained around 100 days post symptom onset, and vaccinated patients were sampled approximately 90 days post-vaccination. Multiple diagnostic assays were employed, including four commercial ELISAs (Abbexa, RayBioTech, FineTest, ProteoGenix) and a multiplex assay (MesoScale Diagnostics (MSD)) measuring five mpox and five smallpox antigens.

Results: Three commercial ELISA kits had low specificity (<50 %). The Proteogenix ELISA targeting the E8L antigen had a 94 % sensitivity and 87 % specificity. The E8L antigen on the MSD assay exhibited the greatest distinction between exposure groups, with 98 % sensitivity and 93 % specificity.

Conclusions: None of the assays could distinguish between mpox-positive and vaccinated samples. The MSD assay targeting the MPXV E8L antigen demonstrated the greatest differentiation between mpox-positive and pre-outbreak negative samples. Our findings underscore the imperative to identify sensitive and specific assays to monitor population-level mpox exposure and infection.