Mpox emergence in Japan: ongoing risk of establishment in Asia

On March 7, 2023, four new mpox (formerly known as monkeypox) cases were reported in Japan by the Ministry of Health, Labour and Welfare, bringing its total case count to 31.1Ministry of HealthLabour and Welfare, JapanMpox press releases.https://www.mhlw.go.jp/stf/seisakunitsuite/bunya/mpox_press-release.htmlDate: 2023Date accessed: March 10, 2023Google Scholar The first imported case in Japan was reported on July 25, 2022, and cases have been sporadically reported since, mostly with a history of travelling abroad or after having been in contact with travellers (appendix pp 4–5). From late January, 2023, cases started being reported more frequently, and most of these individuals did not have a travel history. This continuous reporting of new cases suggests that ongoing local transmission might be occurring in Japan (appendix p 2). The current global mpox outbreak, which was first recognised in May, 2022, has mostly affected men who have sex with men (MSM) in Europe and the Americas (appendix p 3). By the end of 2022, Asia was the least affected region, with no or only up to around 20 cases reported in most countries, including countries with the largest population sizes (eg, China, India, Indonesia, Pakistan, Bangladesh, and Japan). Although the global epidemic trend (primarily representing Europe and the Americas) has been on the decline since August, 2022, the global spread of mpox could re-emerge if local transmission was established in these Asian countries with a large susceptible pool. So far, cumulative cases in countries reaching an apparent end of the epidemic generally range between 0·1% and 1·5% of the estimated MSM population size (appendix p 3). If this empirical pattern was applied to Japan, which has an estimated MSM population size of 0·7 million, an epidemic of up to 10 000 cases might be expected in the absence of successful containment. Compared with many of the previously affected countries, Japan is more closely connected with other Asian countries. The majority of its outbound travel volume is for south, southeast, and east Asian countries with relatively large population sizes (figure; data sources detailed in appendix p 1). Some of these countries have reported new cases in 2023 (both with and without a travel history), with at least one of these cases as of March, 2023, having probably been infected in Japan (appendix p 6). Because of the scarcity of mpox vaccination campaigns in Asia (including Japan), these new infections raise a concern that the global mpox outbreak might be entering another phase—a resurgence formed by the spread in Asia. Modelling studies have suggested that mpox transmission could only be sustained in a relatively small subset of sexual networks,2Endo A Murayama H Abbott S et al.Heavy-tailed sexual contact networks and monkeypox epidemiology in the global outbreak, 2022.Science. 2022; 378: 90-94Crossref PubMed Scopus (18) Google Scholar which makes it unlikely to cause sweeping epidemics,3Murayama H Pearson CAB Abbott S et al.Accumulation of immunity in heavy-tailed sexual contact networks shapes monkeypox outbreak sizes.medRxiv. 2022; (published online Nov 15.) (preprint).https://doi.org/10.1101/2022.11.14.22282286Google Scholar such as the COVID-19 pandemic. However, challenges in control due to its sexually associated nature (eg, privacy and stigmatisation) and the disproportionate impact on vulnerable populations should not be underestimated. Rapid and intensive initial response as well as clear and balanced messaging will be key to early containment in Japan. The international community needs to be on alert and to cooperate to prepare for possible consequences, with support for response and vaccine roll-out especially in low-income and middle-income countries in Asia. AE is supported by the Japan Society for the Promotion of Science (JSPS) Overseas Research Fellowships, JSPS Grants-in-Aid KAKENHI (22K17329), foundation for the Fusion Of Science and Technology, and Japan Science and Technology Agency Precursory Research for Embryonic Science and Technology (JPMJPR22R3). S-mJ is supported by the Centers for Disease Control and Prevention Safety and Healthcare Epidemiology Prevention Research Development programme (200-2016-91781). FM is supported by JSPS Grants-in-Aid KAKENHI (20J00793). Editorial note: The Lancet Group takes a neutral position with respect to territorial claims in published maps and institutional affiliations. For more on mpox outbreak data see https://worldhealthorg.shinyapps.io/mpx_global/ For more on mpox outbreak data see https://worldhealthorg.shinyapps.io/mpx_global/ Download .pdf (.64 MB) Help with pdf files Supplementary appendix