Maximizing the impact of limited vaccine supply under different early epidemic conditions: a 2-city modelling analysis of monkeypox virus transmission among men who have sex with men

Background:The current global monkeypox virus (MPXV) outbreak has disproportionately affected gay, bisexual and other men who have sex with men (GBMSM). Given that many jurisdictions have been faced with limited supplies of MPXV vaccine, we aimed to explore optimal vaccine allocation between 2 linked GBMSM transmission networks over a short-term time horizon, across several epidemic conditions.Methods:We constructed a deterministic compartmental MPXV transmission model. We parameterized the model to reflect 2 representative, partially connected GBMSM sexual networks ( cities), using 2022 data from Ontario. We simulated a roll-out of 5000 vaccine doses over 30 days that started 45 days after epidemic seeding with 10 imported cases. Within this model, we varied the relative city (network) sizes, epidemic potentials (R₀), between-city mixing and distribution of seed cases between cities. For each combination of varied factors, we identified the allocation of doses between cities that maximized infections averted by day 90.Results:Under our modelling assumptions, we found that a limited MPXV vaccine supply could generally avert more early infections when prioritized to networks that were larger, had more initial infections or had greater R₀. Greater between-city mixing decreased the influence of initial seed cases and increased the influence of city R₀ on optimal allocation. Under mixed conditions (e.g., fewer seed cases but greater R₀), optimal allocation required doses shared between cities.Interpretation:In the context of the current global MPXV outbreak, we showed that prioritization of a limited supply of vaccines based on network-level factors can help maximize infections averted during an emerging epidemic. Such prioritization should be grounded in an understanding of context-specific risk drivers and should acknowledge potential connectedness of multiple transmission networks.

The emerging outbreak of monkeypox virus (MPXV) worldwide included 1435 cases in Canada as of Oct. 28, 2022.¹ A third-generation replication-deficient smallpox vaccine (Imvamune) has been licensed for use against MPXV and related orthopoxviruses in Canada since 2020, for the purpose of national security.² Shortly after cases were reported in Canadian cities, rapid pre-exposure prophylaxis vaccination efforts were started to help reduce acquisition, infectivity and disease severity among communities disproportionately affected by MPXV, including gay, bisexual and other men who have sex with men (GBMSM).^3–6 However, jurisdictions across Canada and beyond were faced with a limited local supply of vaccines during the first few weeks of the MPXV outbreak.It is well established that prioritizing a limited supply of vaccines to subpopulations with a disproportionately higher transmission risk (i.e., acquisition and/or transmission at the individual level and/or network levels) can maximize infections averted.^5–8 Such networks may have different characteristics that shape the epidemic potential within the network itself.⁹ This potential is often quantified via the basic reproduction number R₀, which reflects the expected number of secondary infections generated by a person who is infected in a fully susceptible population.¹⁰ A network’s connectedness to other networks further shapes if and how many cases are imported by the time vaccine allocation decisions and rollout begin.¹¹We sought to explore the optimal allocation of a fixed supply of MPXV vaccine across 2 partially connected transmission networks (reflecting jurisdictions) of GBMSM (reflecting the community with the most cases of MPXV infection currently) under different epidemic conditions. Specifically, we explored differences between 2 jurisdictions in GBMSM population size, epidemic potential (R₀), imported or seed cases, and connectedness of the 2 jurisdictions. Our goal was to produce fundamental and generalizable insights into the prioritization of MPXV vaccine in the context of interconnected sexual networks, using jurisdictions (cities) within Ontario as an example, to guide policy-makers in allocating scarce vaccines to maximize infections averted.