Thailand Confirms First Case of New, Deadlier Mpox Strain, Clade 1b

Health officials in Thailand said on Thursday that they had confirmed a case of the version of mpox that prompted the World Health Organization to declare a global health emergency. It’s the second time that the new and deadlier version has been found outside Africa.

The announcement of the case in Thailand is likely to stir concerns about the virus spreading more widely, especially after the version was discovered in Sweden last week. Previously the outbreak had been concentrated in the Democratic Republic of Congo.

The version of the mpox virus detected in these recent cases is known as Clade Ib. Health officials are particularly concerned about it because it has a death rate of 3 percent, much higher than the 0.2 percent death rate observed in a 2022 outbreak.

That earlier outbreak was driven by a version called Clade IIb, which is spread predominantly through sexual contact. Men who had sex with men proved to be the most at risk, but behavioral changes and vaccinations curbed the spread.

Clade Ib appears to have spread mainly through heterosexual sex, epidemiologists have said. Another subtype, Clade Ia, has spread through household contact and exposure to affected animals in addition to sexual contact. So far, young children have been the most vulnerable to this subtype.

Thai officials said on Wednesday that the infected person was a 66-year-old European man who worked in an African country with an ongoing outbreak. They did not specify which country. The man, who has a home in Thailand, was not reported to have severe symptoms.