Three California residents have been infected with clade I mpox, a more severe strain of the virus formerly known as monkeypox – marking the first time this type of mpox has spread within the US, health officials said on Friday.
The unrelated cases, identified in Long Beach and Los Angeles county, involve patients who had not recently traveled abroad. All three were hospitalized and are now recovering, according to the California department of public health.
Mpox is a viral infection that often begins with flu-like symptoms such as fever, chills and muscle aches. These are usually followed by a rash that starts as small raised spots, then develop into fluid-filled blisters before forming scabs.
The virus has two main genetic groups, or “clades”. Clade I mpox has caused widespread disease and deaths in parts of central and east Africa, most acutely in the Democratic Republic of the Congo. Clade II was the strain behind the 2022 US outbreak, which has circulated at low levels since.
“The presence of clade I needs careful evaluation,” said Seth Blumberg, an associate professor of medicine at the University of California, San Francisco’s division of infectious diseases and hospital medicine.
“I don’t think the historical results we’ve seen from Africa are directly translatable, but we need to monitor these cases closely.”
Dr Rita Nguyen, assistant state public health officer, said in Friday’s statement that “clade I mpox cases can be severe” and the “risk of severe disease and hospitalization are highest for people with weakened immune systems.”
“It’s critical to protect yourself by getting both doses of the mpox vaccine if you or your sex partner(s) may be at risk for mpox,” she said.
Anyone can contract mpox through close physical contact, including sex. Past outbreaks of clade II primarily affected gay and bisexual men and transgender or non-binary people who have sex with men. California’s public health department now believes person-to-person spread of clade I is occurring in California, mainly affecting these communities.
In Long Beach, officials announced the first case on Tuesday.
The Centers for Disease Control and Prevention (CDC) has reported six prior US clade I cases, all linked to travel abroad. The first was detected in San Mateo county in 2024. Officials said the overall risk to the public remains low, and casual contact in settings like offices or public transit is unlikely to spread the virus.
The Jynneos vaccine protects against both clade I and II mpox. It is a two-dose series given a month apart, with full immunity reached two weeks after the second dose. No booster is currently recommended.
Vaccination doesn’t completely eliminate the risk of contracting mpox, but experts say it helps reduce severity if a person were to contract the disease.
“The vaccines are completely protective,” Blumberg said. “Vaccines can prevent the severity of disease. If you were to get mpox, you would be less likely to need advanced care.”
The CDC said it is not known how long vaccine protection might last, or if protection might decrease over time, according to its website. The public health agency is analyzing the current data and conducting studies to better understand how the Jynneos vaccine works amid the current mpox outbreak, as well as how long protection might last.
