WASHINGTON — Health officials have announced what could be the first instances of domestic transmission of a severe mpox strain within the United States. This marks a concerning development as the virus appears to spread beyond travelers returning from abroad.

In California, authorities confirmed a case involving a Long Beach resident diagnosed with “clade I mpox.” On Thursday, Los Angeles officials reported a similar case, as noted by the Associated Press.

The identities of the individuals involved have not been disclosed. Both required hospitalization but are now isolating and recovering at home. “Currently, our investigations have not uncovered any connection between the two cases,” stated Jennifer Ann Gonzalez, acting public affairs officer for the Long Beach health department.

Although Long Beach is part of Los Angeles County, it operates its own city health department. Local investigators have yet to identify any close contacts who have traveled internationally, nor have they confirmed any additional cases. According to Nora Balanji, the Long Beach department’s communicable disease coordinator, a few close contacts of the patient have been vaccinated as a precaution.

This is not the first occurrence of clade I mpox in the U.S. Prior to this week, six cases were reported, all linked to international travel. The Centers for Disease Control and Prevention note that the disease was first reported in the U.S. in 2024.

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. 

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.