All six previous U.S. cases of this version of mpox were among international travelers who were believed to have been infected abroad.
WASHINGTON — The first cases of what may be in-country transmission of a severe strain of mpox has been reported in the U.S., health officials said this week.
California health authorities said that a Long Beach resident had a confirmed case of what is known as “clade I mpox.” Los Angeles officials on Thursday reported a second, similar case, according to the Associated Press.
Neither of the California patients were identified. Both required hospitalization, but are now isolating and recovering at home. But “at this point in our investigations, we have not identified any association between the two cases,” Long Beach health department acting public affairs officer Jennifer Ann Gonzalez said.
Long Beach is located in Los Angeles County but has its own city health department. Investigators there say they have not found a close contact who traveled abroad, nor have they confirmed additional cases. A few of the person’s close contacts have been given a vaccine, said Nora Balanji, the Long Beach department’s communicable disease coordinator.
It’s not the first U.S. case of clade I mpox. Six cases of the disease have been documented in the U.S. before this week, but all six were from international travelers who officials believe were infected while abroad. The disease first made its way to the U.S. in 2024, according to the Centers for Disease Control and Prevention.
How dangerous is the new mpox strain?
Clade I mpox is considered more severe and dangerous than the more common clade II variant, according to the Los Angeles County Public Health Department.
Milder symptoms can include fever, chills and body aches. In more serious cases, people can develop lesions on the face, hands, chest and genitals.
The clade I variant is also significantly more deadly than its more common counterpart. Johns Hopkins University notes that the mortality rate for clade I mpox is somewhere between 1% and 10%, while the mortality rate for clade II infections has ranged from less than 1% to 4%.
But public health officials say the risk to the public is low, and that the disease can be combatted effectively with a vaccine.
What is mpox?
Mpox — which was renamed from monkeypox in 2022 over racism concerns — is a rare disease caused by infection with a virus that is in the same family as the one that causes smallpox. It is endemic in parts of Africa.
One version of the virus — called clade II — was the source of an international health crisis in 2022, when infections escalated in dozens of countries, mostly among men who have sex with men. At one point, the U.S. was averaging close to 500 cases per day.
The infections were rarely fatal, but many people suffered painful skin lesions for weeks. Those outbreaks waned later that year, thanks in part to the Jynneos vaccine made by Bavarian Nordic.
The other version — known as clade I — likewise can spread through sex, but also through other forms of contact. In Africa it has infected a broader range of people, including children.
A newer form of the clade I virus has been widely transmitted in eastern and central Africa. The World Health Organization declared the situation a public health emergency, but last month it said the problem had waned enough that it was no longer an international emergency.
Still, “it’s concerning if this virus has come here and now is starting to be transmitted from person to person,” said Dr. William Schaffner, an infectious diseases expert at Vanderbilt University.
The Associated Press contributed to this story.