Gregory Gan

Background: The ongoing 2023–2024 mpox outbreak in several African countries, driven by the novel Clade Ib strain, has resulted in imported cases reported in Sweden, Thailand, and India. The potentially high transmissibility of this new strain and shifts in transmission modes may make territories in Asia, which were minimally affected by previous mpox waves, susceptible to community-wide transmission following importation. While this highlights the importance of early preparedness, current knowledge of the virus’s transmission dynamics remains too limited to effectively inform policy-making and resource planning. Methods: A compartmental model was constructed to characterise the potential mpox transmission dynamics. Importation-triggered outbreaks were simulated in 37 Asian cities under scenarios with one, three, and five initial local infections. The impacts of various non-pharmaceutical interventions (NPIs), including isolation and quarantine, were projected and compared. Findings: Our simulations revealed substantial disparities in outbreak sizes among the 37 Asian cities, with large-scale outbreaks expected in territories with a high proportion of sexually active individuals at risk or low immunity from smallpox vaccination. Total case count in one year following initial local infections would increase linearly with initial infection size. In the scenario with three initial local infections, up to 340 cases per million residents were expected without interventions. Isolation for diagnosed cases was projected to lower the outbreak size by 43.8% (interquartile range [IQR]: 42.7%–44.5%), 67.8% (IQR: 66.5%–68.9%), 80.8% (IQR: 79.5%–82.0%), and 88.0% (IQR: 86.8%–89.1%) when it reduced interpersonal contacts by 25%, 50%, 75%, and 100%, respectively. Quarantining close contacts would contribute to a further decrease in cases of up to 22 percentage points over one year. Interpretation: The potential mpox outbreak in an Asian setting could be alleviated through strong surveillance and timely response from stakeholders. NPIs are recommended for outbreak management due to their demonstrated effectiveness and practicability. KEY MESSAGES: What is already known on this topic: Clade Ib monkeypox virus (MPXV) is circulating in the Democratic Republic of the Congo (DRC) and its neighbouring countries, with imported cases being identified globally. Meanwhile, evidence indicates that the virus can be transmitted through both sexual and non-sexual routes, raising concerns about its potential spread in the general population. To prevent a global outbreak, the World Health Organization (WHO) suggested countries around the globe to prepare in advance. What this study adds: Our simulations quantified the potential disease burden of an mpox outbreak triggered by a once-time importation event in 37 major Asian cities with varying pre-existing immunity levels and populations at higher risk due to frequent sexual activities. The compartmental modelling framework developed in this study also projected the effectiveness of diverse non-pharmaceutical intervention (NPI) strategies in outbreak control, providing policy-makers with guidance for effective public health crisis management. How this study might affect research, practice or policy: An importation-triggered mpox outbreak can be substantially mitigated with powerful disease surveillance and a prompt response of the stakeholders, but may also lead to severe consequences with high morbidity and mortality if not addressed in time in cities with a large highly sexually active population. Various NPIs, particularly isolating infected cases, are recommended for curbing the disease outbreak due to their feasibility and effectiveness in the Asian setting.

The transmission dynamics of the recent mpox outbreak highlights the lack of infrastructure available to rapidly respond to novel STI outbreaks, of which Asia and Oceania remains particularly susceptible. Here, we simulate outbreaks in this setting and propose the use of pre-emptive vaccination within the men who have sex with men (MSM) community before the arrival and establishment of the virus. Using data driven heterogeneous sexual contact networks, we simulated outbreaks of mpox in Singapore, Hong Kong, and Sydney. An individual based SEIR compartmental model was used to simulate epidemic trajectories and the impact of different vaccination uptakes was assessed in their ability to avert or suppress outbreaks upon the arrival of mpox within the MSM populations. The highly dense sexual networks of Singapore and Sydney experience rapid outbreaks, with infection peaks occurring at day 41 and 23 respectively, compared to Hong Kong which occurs at day 77. Across the simulations with no vaccination, 68.2%–89.7% of the MSM community will become infected with mpox across the different cities, over a simulation period of 1 year. By implementing vaccination strategies, the infection rate across the cities can be reduced to as low as 3.1% of the population (range: 3.1%–82.2%) depending on the implementation and uptake of the vaccine. Vaccination is also extremely effective in slowing the start of the epidemic, delaying the epidemic peak by 36–50 days in Hong Kong, or even preventing the outbreak of mpox. With extremely dense and well-connected sexual contact networks, where 65.2%–83.2% of the population are connected to a super-spreader in the different contact networks, pre-emptive or immediate vaccination upon identification of the first case is strongly recommended to help better manage the outbreak of mpox and prevent potential straining of healthcare systems.