Mpox vaccination was promoted after a severe outbreak was discovered in San Francisco
California health officials are urging high-risk residents to get vaccinated against mpox after a highly virulent strain was found 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 case occurred in an unvaccinated adult who was in close contact with someone who had traveled internationally, the San Francisco Department of Public Health said. It was the seventh case of Clade I mpox in California, and the resident had to be hospitalized for treatment.
Clade I was first detected in California in November 2024, in San Mateo County, and additional cases were found in Los Angeles County in October.
One case of Clade I was identified in LA County in January, but there has been no further spread of the disease, according to the LA County Department of Public Health.
Most cases of mpox, formerly known as monkeypox, which began to spread in California and the US in 2022 have been of the Clade II type.
Clade I infections may be of greater concern than Clade II because they can cause severe illness and spread easily, through close contact such as sex, massage or hugging, health officials said.
While Clade I cases remain rare, California officials say Clade II cases are on the rise — case rates are higher than those seen two years ago. So far this year, there have been 14.5 mpox cases per week, compared to a rate of 3.4 in the comparable period in 2025 and 5.8 in the comparable period in 2024.
Most of the infected people have not been vaccinated. The vaccine reduces the chances of getting any type of mpox, as well as suffering serious illness if infected. Those most at risk are people with weak immune systems.
“This reinforces how important it is that people at high risk get both doses of the mpox vaccine,” said Dr. Erica Pan, director of the California Department of Public Health and chief public health officer, in a statement. “With summer trips and big events approaching, now is the right time to protect yourself if you or your sexual partner could be at risk.”
Those who are considered to be at high risk of mpox include men who have sex with men; transgender, non-binary or gender-queer people; people with HIV; people who are eligible to take medications that reduce the risk of contracting HIV; those traveling to areas of the world with outbreaks of Clade I mpox who expect to have intercourse while traveling; people planning to attend a commercial sex event; or those who have a sexual partner with the above risks.
Vaccination is also recommended for people at risk of occupational exposure, such as certain laboratory workers.
The defining symptoms of mpox “include a rash or unusual sores that look like pimples or pus-filled blisters on the face, body and genitals, fever, chills, headache, muscle aches or swollen lymph nodes,” according to the LA County Department of Public Health.
Mpox is less likely to spread through direct contact, or in close quarters such as on an airplane or in an office or store, state health officials said.
People with mpox symptoms should contact their health care provider immediately and ask for a test. People exposed to mpox should be vaccinated as soon as possible and before symptoms appear, state health officials say. People with empox should stay at home and avoid contact with other people until it heals.
