Abstract Introduction Orthopoxvirus immunization has become increasingly important following the global outbreak of mpox in 2022. Universal smallpox vaccination was discontinued in the 1970s, and the emergence of mpox as a public health threat likely corresponds to a growing non-immune population. Methods Saliva and clinical data, including vaccine and infection history, was collected from attendees at three queer-focused sex-positive events hosted in San Francisco, California between July 2023 and November 2023 after informed consent. Antibody titers (IgG) in saliva were measured using a multiplex orthopoxvirus assay measuring five vaccinia virus (VACV) antibodies and five MPXV antibodies. Primary outcome was antibody level between participants who had been vaccinated, previously infected, or naïve by self-report, compared using Welch’s t-test. A secondary analysis compared titer to number of vaccine doses. All analyses were performed using R (version 4.4.1) in RStudio (version 2024.09.0 + 375). Results Ninety participants provided a saliva sample, of which 69 (77%) were analyzable for our primary outcome. Four were excluded because no antibodies could be recovered, nine had dried up in transport, and eight had uncertain exposure history. Ten of these participants reported prior MPXV infection, 58 reported prior vaccination but no prior infection, and six reported no prior vaccination or infection. Consistent with serum studies, the performance of each antibody varied based on exposure status. Antibody titer for MPXV A29L and VACV A27L could best discriminate between prior infection and/or prior vaccination when compared with those with no prior exposure (Figure 1a, MPXV A29L exposed mean 186.3 vs unexposed mean 12.7 with p = 0.012; VACV A27L exposed mean 194.4 vs unexposed mean 13.7 with p = 0.017). Most (40/53) vaccinated individuals received the recommended two-vaccine series, but saliva titers were markedly higher for the individuals who received three or more vaccines (Figure 1b). Discussion Our study demonstrates the feasibility of collecting saliva at community-based gatherings in an outbreak situation, with 77% usable. We found differences in orthopoxvirus saliva serology to differentiate between previously infected, previously vaccinated, and unexposed individuals. The high titers in participants who received three or more vaccines suggest a booster dose may be useful, particularly as mpox cases have been reported in vaccinated individuals. A limitation of our study is the relatively small number of unvaccinated participants in this highly healthcare-engaged population. Given the ease of collection, saliva serology may be a useful tool for studies of orthopoxvirus epidemiology, as well as for other emerging pathogens. This abstract is funded by: None
Contag et al. (Fri,) studied this question.