California confirms first US case of more severe mpox strain — but risk to public remains low

(SAN MATEO COUNTY, Calif.) — Health officials in California have confirmed the first domestic case of a more severe strain of mpox in a traveler from Eastern Africa, according to the U.S. Centers for Disease Control and Prevention.

The risk to the public remains low, according to the agency. Health officials are working to identify any people who may have been exposed to the person who recently traveled from Eastern Africa and was treated shortly after returning to the U.S. at a local medical facility, the CDC said.

Casual contact — including during travel — is unlikely to pose significant risks for transmission, the agency noted.

This is the first confirmed case in the U.S. of a strain, or clade, of the virus called clade 1b.

Another strain, clade 2b, was primarily responsible for the global outbreak in 2022. During the outbreak, mpox primarily spread through close sexual contact — and a strong vaccination effort was credited with helping slow the surge of cases. This less severe version continues to circulate in the U.S at low levels. The CDC does not currently recommend booster shots for those who are considered high-risk and already fully vaccinated.

Clade 1b was recently found in September in the Congo, likely responsible for a growing outbreak in Africa that led the World Health Organization to declare a global health emergency. Early data shows that this clade is more severe and may be spreading through other contact routes in households and often to children.

People with mpox, which was formerly known as monkeypox, often get a rash that can be located on hands, feet, chest, face, mouth or near the genitals, the CDC said.