(P213) Lack of detectable antibodies to mpox in infected individuals

Background: On July 23, 2022, the World Health Organization (WHO) declared mpox a Public Health Emergency of International Concern (PHEIC). Since then there have been almost 85,000 cases globally, with over 30,0000 in the USA. Of these, 99% have been in men with 94% reporting recent intimate male-to-male contact (MSM). Asymptomatic carriage has been reported as a potential mechanism for the extensive dissemination of mpox globally. Seroprevalence studies can help determine the true burden of epidemic but also identify “hidden” populations for mitigation strategies. However, it is unclear if the potential cross-reactivity to small pox (for vaccinated individuals) is problematic. Our aim was to determine the performance of antibody assays for identifying individuals infected with mpox to then perform sero surveys on general and high-risk populations.

Methods: Plasma samples were obtained from 8 previously symptomatic PCR-positive patients, and 144 PCR-negative at Johns Hopkins Hospital (JHH), Baltimore, USA. For the 8 positive patients, samples were collected a mean of 100 days (range 55-134) post PCR+ date. A majority (7/8) of individuals were persons living with HIV (PLWH). PLWH had a mean CD4 count of 554 (range 360-1025), with 6/7 (85.7%) having HIV-1 RNA < 200 copies/ml. Negative control samples were obtained from excess discarded samples from JHH Emergency Department patients in 2021. Three commercial ELISA kits (Abbexa MPXV IgG [Cambridge UK], RayBioTech MPXV IgG [Peachtree GA, USA], and FineTest IgG [Wuhan, China]) were performed according to the manufacturer’s protocol, and results reported in normalized optical density (ODn). All three ELISA purported to measure antibodies to the mpox A30L protein.

Results: All positive and negative controls provided by the manufacturers provided the expected results. The mean (IQR) ODn for known infected individuals was 0.13 (0.05, 0.16), 0.12 (0.06, 0.17), 0.16 (0.11, 0.16) respectively. These values were near identical to known uninfected controls 0.17 (0.13, 0.25), 0.13 (0.11, 0.19), 0.16 (0.09, 0.23) (Figure).

Conclusion: None of the eight individuals had detectable antibodies to mpox as measured by the three commercial ELISAs evaluated. Serosurveys to determine the prevalence of mpox are not feasible using ELISAs testing for antibodies to the A30L protein of mpox.