Modeling the impact of prioritizing first or second vaccine doses during the 2022 mpox outbreak

Background Early in the 2022 mpox outbreak, vaccine doses and administrative capacity were limited. The US recommendation was to administer two doses of the JYNNEOS® vaccine 4 weeks apart. However, because of the limited vaccine supply and high demand, New York City (NYC) prioritized administration of first doses to reach a larger portion of the impacted population as quickly as possible. We estimated mpox cases averted compared to strategies that prioritized 2-dose vaccination for a smaller portion of the population. Methods We fit a dynamic network transmission model to incident mpox cases reported by NYC, as well as to first and second vaccine doses administered from May 2022 through March 2023. Model output consisted of predicted cases over time when vaccine doses were administered with the ‘first-dose priority’ strategy, compared with counterfactual simulations where individuals were either pre-allocated full courses of the vaccine (‘second-dose priority’ strategy), or not pre-allocated doses, but where doses were administered to those eligible for a second dose ahead of those waiting for a first dose (‘intermediate’ strategy). Results We estimate that NYC’s ‘first-dose priority’ strategy averted 81% [IQR:75%–86] of potential mpox cases. Their ‘first-dose priority’ strategy was more effective than alternatives, averting 3.0% [IQR:1.2%–4.5%] more cases than the ‘intermediate’ strategy, and 9.5% [IQR:7.7%–12%] more cases than the ‘second-dose priority’ strategy. Conclusions A focus on widespread, 1 dose vaccination during future mpox outbreaks can reduce cases and limit transmission in scenarios of limited vaccine supply, limited vaccine administration capacity, or increased demand.