California has detected the country’s first case of a new mpox strain that is spreading from person to person. The infected individual had recently traveled from Eastern Africa, where multiple countries are battling transmission of this virus.
The individual was treated in a health facility in San Mateo County and is in isolation at home, the California Department of Public Health announced in a statement released Saturday. In a separate statement, the Centers for Disease Control and Prevention said the person is not currently being treated with anti-mpox medication, which may be a signal that the infection is mild.
The version of the virus the person is infected with is known as clade Ib, an offshoot of clade I viruses that traditionally spread to people from an animal source in countries in East Africa where the virus lives in nature. It is different from the clade IIb virus that is responsible for the large international mpox outbreak that began in 2022.
California health authorities said at this time there is no evidence that the infected person had transmitted the virus to anyone else.
“People who had close contact with this individual are being contacted by public health workers, but there is no concern or evidence that mpox clade I is currently spreading between individuals in California or the United States,” the statement said.
The World Health Organization declared the ongoing person-to-person spread of mpox a Public Health Emergency of International Concern in August. The 2022 outbreak was also declared a PHEIC.
Specimens from the California case have been sent to the CDC for further analysis.
Person-to-person spread of clade Ib viruses was first detected in September 2023 in the Democratic Republic of the Congo. While most of the transmission of this virus is occurring in African countries, travel-related cases have been detected in Germany, India, Kenya, Sweden, Thailand, Zimbabwe, the United Kingdom, and now the United States.
The CDC said it currently considers the risk posed to the U.S. population by the clade Ib outbreak in Central and Eastern Africa to be low, a position it said is supported by simulation exercises it ran.
“These simulations indicate that close-contact transmissions within and between households are unlikely to result in a large number of mpox clade I cases in the United States,” the statement said. “Additionally, in Sweden, Thailand, Germany, and India there was no apparent onward spread of the virus and the onward spread in the United Kingdom has been limited to close, household contacts so far.
So far this year more than 115,000 mpox cases — both clade I and clade II — have been reported by at least 123 countries. Of those infections, at least 255 have been fatal, according to a database maintained by the WHO.
Mpox virus is related to smallpox, a disfiguring and often deadly virus that was declared eradicated in 1980. (Smallpox is the only human virus that has been eradicated.) Like smallpox, mpox triggers a blistering rash that leaves scars. It can also be deadly, but the case fatality rate is significantly lower than that associated with smallpox.
Historically most cases were thought to have been contracted when someone — often children or hunters — had contact with infected animals in the parts of Africa where the viruses are found. Occasionally there was onward spread from a person infected in this way, either a family member or health worker who had contact with the infected person’s lesions, or through contact with contaminated bedding.
But since 2022 it has been apparent that sexual transmission of the viruses is occurring, and has been for some time. The clade IIb outbreak was largely driven by transmission in communities of gay, bisexual, and other men who have sex with men. Clade Ib spread has been associated with men who have sex with men as well, but also sex workers and their clients, and household contacts of these individuals. There are vaccines that protect against mpox, primarily Bavarian Nordic’s Jynneos vaccine. But global production of these products is limited.