Three people have been hospitalised in the US after contracting "severe" cases of mpox - despite not having travelled overseas.
Health authorities in California have confirmed that this is the first time this type of mpox, formerly known as monkeypox, has spread in the States.
The patients, who had no recent history of international travel, were diagnosed in Long Beach and Los Angeles County in separate, unconnected incidents.
All previous known cases of clade I in America had all been associated with foreign travel.
The trio required hospital treatment - but are currently in recovery, according to the California Department of Public Health.
Dr Rita Nguyen, an assistant state public health officer, warned that "clade I mpox cases can be severe" and stressed that "the risk of severe disease and hospitalisation 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 added.
The Jynneos vaccine defends against both variants of mpox through a two-dose regimen administered four weeks apart, with complete protection developing a fortnight after the final injection.
All previous known cases of mpox clade I in America had all been associated with foreign travel
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"Vaccines can prevent the severity of disease. If you were to get mpox, you would be less likely to need advanced care," said Seth Blumberg, an associate professor of medicine at the University of California, San Francisco's division of infectious diseases.
The initial case of clade I mpox in America was identified in San Mateo County during 2024.
Five subsequent cases followed, though unlike this recent mini-outbreak, all were connected to international travel.
Officials from the Centres for Disease Control and Prevention have stressed that the general risk to the public remains minimal.
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At the moment, transmission through casual encounters in workplaces or on public transport are considered improbable.
Historically, mpox was known to have been occasionally passed on from infected animals to humans, with humans sometimes passing it on to other people that they had close contact with.
The virus spreads through intimate physical contact, including sexual activity, with symptoms typically beginning as fever, chills and muscle pain before progressing to a distinctive rash.
These lesions initially appear as small raised marks, then transform into blisters containing fluid before eventually developing into scabs.
Mpox lesions initially appear as small raised marks, then transform into blisters containing fluid before eventually developing into scabs
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In California, state health authorities have found that transmission rates are at their highest among gay and bisexual men, along with transgender and "non-binary" individuals who engage in sexual activity with men.
Exactly two months ago, Californian officials revealed that a person in the state had tested positive for the plague in an extraordinarily rare infection.
The individual may "have been bitten by an infected flea", they revealed.