How to utilise the limited supply of vaccines for Mpox control in Thailand among high-risk GBMSM

Abstract

Mpox re-emerged globally in 2022, disproportionately affecting gay, bisexual, and other men who have sex with men (GBMSM). In 2024, Thailand became the first Asian country to detect Clade Ib Mpox, prompting urgent decisions on deploying a limited supply of 3000 vaccine doses. However, evidence on the comparative effectiveness of different vaccine allocation and behavioural strategies in this context remains scarce. We developed a deterministic compartmental model of Mpox transmission among high- and low-risk GBMSM, calibrated to Thailand's national surveillance data (January 2023–May 2025). The model simulated a range of hypothetical scenarios under a constrained supply of 3000 vaccine doses, distributed either over a short 5-month period or extended across the 28-month epidemic horizon. We evaluated pre-exposure prophylaxis ($PrEP$), post-exposure prophylaxis ($PEP$), dose-sparing regimens, and mixed allocations of the two approaches. Each strategy was examined under alternative rollout timings (early vs. supply-delayed) and in combination with behaviour change, represented as reductions in sexual activity during symptomatic periods. The model reproduced Thailand's epidemic trajectory. Our simulations suggested that early PrEP rollout would have yielded the greatest reduction in incidence, particularly among high-risk GBMSM. PEP strategies would have had a modest impact overall, though single-dose sparing with delayed rollout (months 5–9) would have been notably effective as the epidemic peak occurred during this period. Mixed PrEP and PEP approaches would have produced intermediate benefits, while behaviour change alone significantly would have lowered transmission. Combining PEP with even modest behavioural changes further enhanced prevention and helped reduce spillover into low-risk groups. Under constrained vaccine supply, dose-sparing and mixed vaccination strategies could improve overall coverage and impact, especially when paired with behavioural changes. Integrating flexible and context-specific vaccination approaches with realistic behavioural modifications offers the best potential for Mpox control in Thailand and similar settings.