Health officials in California have confirmed three cases of the more severe strain of mpox known as Clade I, occurring in residents without recent travel history—marking the first known cases in the United States believed to be locally transmitted.
Mpox (formerly referred to as monkeypox) is caused by an orthopoxvirus and has two major genetic groups, or clades: Clade II (which drove the 2022 U.S. and global outbreaks) and Clade I, which is historically more severe and is endemic in parts of Central and East Africa.
Clade I has been associated with higher rates of severe illness and death in Africa (fatality rates in some series ranged from about 1 %–10 %) compared with Clade II (fatality rates under 1 % to 4 %).
The three U.S. cases arose in the Los Angeles County/Long Beach area. None of the affected individuals had recent travel to regions where Clade I is known to circulate, suggesting domestic transmission. All three were hospitalised and are reported to be recovering.
Health officials have not yet confirmed a chain of transmission or traced all contacts. The California Department of Public Health and Los Angeles County agencies are working to identify sources and monitor for spread.
Like the earlier Clade II strain, Clade I mpox typically begins with flu-like symptoms—fever, chills, swollen lymph nodes, headache and muscle aches—followed by a characteristic rash that may turn into blisters, then scabs. However, experts note that Clade I may involve more extensive rash distribution, possibly including face and torso, and may lead to more severe complications such as sepsis or encephalitis, especially in immunocompromised individuals.
Transmission occurs through close physical or intimate contact, including body fluids, skin lesions, shared bedding or clothing, and possibly through non-sexual household contact. Casual contact (such as sharing a room or passing someone in public) is considered low risk, though Clade I may have slightly broader transmission potential.
Until now, all Clade I mpox cases in the U.S. had been linked to international travel. The Californian cases are the first without travel history, raising concern that this strain may have established local chains of transmission in the United States.
Despite this, the overall risk to the general public remains low, according to the Centers for Disease Control and Prevention (CDC) and state public health agencies.
While anyone can contract mpox, previous outbreaks and current surveillance suggest that certain groups are at greater risk, including:
People with weakened immune systems.
Men who have sex with men (MSM) and others within dense contact networks.
Individuals exposed to skin lesions or body fluids of infected persons through close contact.
Given the nature of Clade I and the current cases, health officials are particularly encouraging vaccination and testing among higher-risk groups.
The currently available vaccine in the U.S., Jynneos, is recommended for people at increased risk of mpox exposure. While not 100 % protective, it significantly lowers the likelihood of severe disease and hospitalisation.
General preventive measures:
Avoid close physical or intimate contact with anyone who has a rash or unexplained lesions.
Do not share bedding, towels or clothing with an infected person.
Seek medical evaluation if you develop flu-like symptoms followed by a rash.
Ensure vaccination if you belong to a high-risk group or have recent exposure.
Health authorities stress that prompt testing, contact tracing and vaccination remain key to preventing further spread.
The discovery of potential local transmission of Clade I mpox in the U.S. signals a need for enhanced surveillance, laboratory testing and awareness among clinicians. Although the general risk is low, this event highlights how an imported variant can adapt and spread domestically. Health systems must ensure rapid access to diagnostics and vaccine supply.
For the public, the best approach is informed caution—not alarm. Because casual interaction remains unlikely to cause spread, standard hygiene practices and awareness of symptoms are sufficient for most people.
1. Centers for Disease Control and Prevention (CDC), Monkeypox (Mpox) Situation Summary, last reviewed September 30, 2025, https://www.cdc.gov/monkeypox/situation-summary/index.html.
(Rh/Eth/TL/MSM)
