Health officials have reported that three individuals in California have contracted the clade I strain of mpox, a more virulent type of the virus previously known as monkeypox. This is the first recorded instance of this strain spreading domestically within the United States.

The cases, discovered in Long Beach and Los Angeles County, are not linked and involve patients who have not traveled internationally recently. All three were hospitalized but are now on the path to recovery, according to the California Department of Public Health.

Mpox is a viral illness that typically starts with symptoms similar to the flu, such as fever, chills, and muscle pain. These initial signs are generally followed by a rash that evolves from small, raised spots to fluid-filled blisters, eventually forming scabs.

The virus is divided into two primary genetic categories, known as “clades.” Clade I has been responsible for significant outbreaks and fatalities in central and eastern Africa, particularly in the Democratic Republic of the Congo. In contrast, Clade II was the cause of the mpox outbreak in the United States in 2022, which has been present at low levels since then.

“The detection of clade I warrants thorough investigation,” stated Seth Blumberg, an associate professor specializing in infectious diseases and hospital medicine at the University of California, San Francisco.

“While the outcomes observed in Africa may not directly apply here, it is crucial to keep a close watch on these cases,” he added.

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.