September 16, 2025, the Ministry of Health, Labour and Welfare and Kobe City announced that a woman in her 20s, who visited a medical facility in Kobe, tested positive for the Clade Ib strain of mpox, previously known as monkeypox. This is the first time this strain has been identified in Japan.
The woman, who sought medical attention on September 12 for symptoms including rash and fever, was confirmed to have the Clade Ib strain through genetic testing. She has a history of travel to Africa, and authorities believe she contracted the virus there. Her condition is stable, and Kobe City reports no immediate concerns about the virus spreading from this case.
Mpox spreads through contact with infected bodily fluids or blood. The Clade Ib strain, which has been circulating in central Africa, particularly the Democratic Republic of Congo, since 2023, differs from the Clade IIb strain that spread globally in 2022. According to the U.S. Centers for Disease Control and Prevention, Clade Ib has caused over 40,000 infections and more than 100 deaths in Congo since last year. However, with proper treatment, its fatality rate is below 1%, and no deaths have been reported outside Africa.
The main transmission route for mpox is sexual contact, though close household contact can also spread the virus. Symptoms appear after a 1–2 week incubation period, starting with fever, headache, or muscle pain, followed by rashes on the body or genital area. Recovery typically occurs within 2–4 weeks with proper treatment, though individuals with weakened immune systems are at higher risk of severe illness. Studies show smallpox vaccines can prevent mpox infection if administered within 4 days of exposure or reduce severity if given within 14 days. Japan has a system in place to provide vaccines to those potentially exposed.
The World Health Organization declared a public health emergency in 2024 due to the Clade Ib outbreak in Africa but lifted it on September 5, 2025, as cases and deaths declined due to vaccination efforts and other measures. In Japan, 254 mpox cases, including one death, had been reported as of September 12, with this case marking the 255th.
The Ministry of Health, Labour and Welfare stated that Japan has systems for testing, treatment, and vaccination in place. It urged the public not to fear mpox excessively or discriminate against patients, and to seek medical advice for symptoms like fever or rash while practicing basic infection prevention measures.
Mpox, caused by the monkeypox virus, is a zoonotic disease first identified in 1970 in Africa. The Clade Ib strain’s emergence in 2023 raised concerns due to its higher initial fatality rate, but global health responses, including vaccination campaigns, have reduced its impact. Japan’s robust healthcare infrastructure and preparedness, including access to smallpox vaccines effective against mpox, help manage risks. The country’s experience with the 2022 Clade IIb outbreak has informed its current response to ensure public safety.
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