California health officials are urging high-risk residents to get vaccinated against mpox after a potentially more serious strain of the virus was first detected in San Francisco.
The strain in question, known as Clade I, was identified in a San Francisco resident and announced by local health officials this week. The San Francisco Department of Public Health said the case occurred in an unvaccinated adult who had close contact with someone who had traveled internationally. It was the seventh clade I MPOX case in California, and the resident required hospitalization for treatment.
Clade I was first detected in San Mateo County in California in November 2024, and additional cases were identified in Los Angeles County in October.
Another case of clade I was identified in LA County in January, but according to the LA County Department of Public Health, no further spread of the disease was documented.
Most cases of Mpox, formerly known as monkeypox, which began spreading in California and the US in 2022, are clade II strains.
Health officials say clade I infections are potentially more worrisome than clade II because they can cause more severe disease and spread more easily, such as through close personal contact such as sex, massage or hugging.
While Clade I cases are rare, California officials say Clade II cases are on the rise — much higher than the rates seen in the past two years. So far this year, there have been 14.5 cases of mpox per week, compared to 3.4 in the comparable time period in 2025 and 5.8 in the comparable time period in 2024.
Most of the people infected were not vaccinated. The vaccine reduces the chance of getting any type of mumps and suffering serious illness if infected. Those most at risk are those with weakened immune systems.
“This reinforces how important it is for high-risk people to get both doses of the mpox vaccine,” Dr. Erica Pan, director of the California Department of Public Health and state public health officer, said in a statement. “With summer travel and big events coming up, now is the ideal time to protect yourself if you or your sexual partner may be at risk.”
which were considered high risk MPOX includes men who have sex with men; people who are transgender, non-binary or gender-diverse; People who have HIV; People eligible to take medicines to reduce their risk of getting HIV; People traveling to areas of the world with outbreaks of cladistic eye mpox who anticipate sexual contact during travel; People planning to attend a commercial sex event; Or those who have sexual partners with the above risk factors.
Vaccination is also urged for those at risk for occupational exposure, such as some laboratory workers.
According to the LA County Department of Public Health, obvious symptoms of MPOX include “a rash or unusual lesions on the face, body and genitals that look like pimples or pus-filled blisters, fever, chills, headache, muscle pain or swelling of the lymph nodes”.
State health officials say Mpox is unlikely to spread by casual contact, or in close quarters, such as on airplanes or in an office or store.
People who have symptoms of mpox should contact their health care provider immediately and ask to be tested. State health officials say people exposed to mpox should get vaccinated as soon as possible and before symptoms develop. People who have mpox should stay home and avoid contact with other people until their rash has healed.
