(P134) MODEL-BASED ESTIMATES OF ONE-DOSE AND TWO-DOSE JYNNEOS VACCINE EFFICACY AGAINST MPOX USING SURVEILLANCE AND VACCINE ADMINISTRATION DATA ACROSS COUNTIES THAT HAD COMMUNITY TRANSMISSION OF MPOX

Background:
CDC recommends the two-dose JYNNEOS vaccine for gay, bisexual, and other men who have sex with men at increased risk of mpox infection. Estimates of vaccine effectiveness vary greatly from observational studies: 36%(95%CI 22-47) -75%(95%CI 61-84) for the first dose and 66%(95%CI 47-78) -89%(95%CI 44-98) for the second. Potential explanations of this variability include differences in (A) vaccine administration or exposure route, (B) study design, (C) vaccine administration timing relative to outbreak phase between jurisdictions, and (D) the vaccinated and unvaccinated people within jurisdictions (e.g., risk aversion or immunocompetence), among other possibilities.

Methods:
We used mathematical modeling to derive model-based estimates of vaccine efficacy (per exposure reduction in susceptibility) from one (VE1) or two (VE2) doses of JYNNEOS. We used data across the 50 Ending the HIV Epidemic counties in the United States. This deterministic model was integrated into a Bayesian framework to estimate 3 county-specific transmission parameters (effective population size, force of infection, and initial number of imported cases—150 total parameters) and 3 shared outbreak control parameters (VE1, VE2, and temporal behavioral adaptations). Primary model inputs included weekly county-specific case-reports and vaccine-administration data from 10/2022 to 07/2023. We excluded 8 counties where the outbreak dynamics were indistinguishable from stochastic noise (i.e., <100 cumulative cases or <100 cases per 100,000 vaccine-recommended people).

Results:
Our fitted model replicated each county’s outbreak with error less than 8% of the observed cases on average. We estimated that VE1 was 40.1% (95%CI 36.1, 43.8%) and VE2 was 47.9% (95%CI 40.9, 54.7%).

Conclusion:
This study provides the first model-based estimates of vaccine efficacy against mpox acquisition.  These estimates represent highly conservative values since we permitted maximal behavioral adaptation. These model-based estimates of vaccine efficacy can inform preparedness activities for control scenario modeling, determining vaccine coverage goals, conducting qualitative and quantitative risk assessments, and planning and prioritization.