California has discovered the nation’s first case of a new strain of MPox spreading from person to person. The infected person had recently traveled from East Africa, where several countries are battling the transmission of this virus.
The person was treated at 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 medications, which could be a signal that the infection is mild.
The version of the virus that infected the person is known as clade Ib, an offshoot of clade I viruses that traditionally spread to humans from animal sources in countries in East Africa where the virus lives in nature. It is distinct from the clade IIb virus responsible for the major international mpox outbreak that started in 2022.
California health authorities said at this time there is no evidence that the infected person has transmitted the virus to anyone else.
“People who have had close contact with this individual are being contacted by public health professionals, 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 continued personal 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 transmission of clade Ib viruses was first detected in September 2023 in the Democratic Republic of Congo. Although most of the transmission of this virus occurs in African countries, travel-related cases have been discovered in Germany, India, Kenya, Sweden, Thailand, Zimbabwe, the United Kingdom and now the United States.
The CDC said it currently considers the risk to the U.S. population from the clade Ib outbreak in Central and East 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. “In addition, there was no apparent further spread of the virus in Sweden, Thailand, Germany and India and further spread in the UK has so far been limited to close contacts with households.
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 a database maintained by WHO.
The Mpox virus is related to smallpox, a disfiguring and often fatal virus that was declared eradicated in 1980. (Smallpox is the only human virus that has been eradicated.) Like smallpox, mpox causes a blister-like rash that leaves scars. It can also be fatal, but the mortality rate is significantly lower than that of smallpox.
Historically, it was believed that most cases were contracted when someone – often children or hunters – had contact with infected animals in the parts of Africa where the viruses are found. Occasionally, further spread has occurred through a person infected in this way, either a family member or a healthcare worker who had contact with the infected person’s lesions, or through contact with contaminated bedding.
But as of 2022, it is clear that sexual transmission of the viruses is happening, 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. The spread of Clade Ib is associated with men who also have sex with men, but also with sex workers and their clients, and with household contacts of these individuals. There are vaccines that protect against mpox, mainly the Jynneos vaccine from Bavarian Nordic. But global production of these products is limited.