California urges mpox vaccination after more-severe strain 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 case occurred in an unvaccinated adult who had close contact with someone who had traveled internationally, the San Francisco Department of Public Health said. It was the seventh Clade I mpox case in California, and the resident needed to be hospitalized for treatment.

Clade I was detected for the first time in California in November 2024, in San Mateo County, and additional cases were identified in Los Angeles County in October.

Another case of Clade I was identified in L.A. County in January, but there was no further documented spread of the disease, according to the L.A. County Department of Public Health.

Most cases of mpox, formerly known as monkeypox, that began spreading in California and the U.S. in 2022 have been the Clade II strain.

Clade I infections are potentially more concerning than Clade II because they may cause more severe illness and spread more easily, such as through close personal contact like sex, massage or cuddling, health officials say.

While Clade I cases remain rare, California officials say Clade II cases are on the rise — with case rates well above those seen in the last two years. So far this year, there have been 14.5 cases of mpox per week, compared with a rate of 3.4 over the comparable time period in 2025 and 5.8 for the comparable time period in 2024.

Most of the people who were infected had not been vaccinated. The vaccine lowers the chance of getting either strain of mpox, and of suffering from severe illness if infected. Those at highest risk are people with weakened immune systems.

“This reinforces how important it is for people at higher risk to get both doses of the mpox vaccine,” Dr. Erica Pan, the director of the California Department of Public Health and state public health officer, said in a statement. “With summer travel and large events approaching, now is the ideal time to protect yourself if you or your sex partner may be at risk.”

Those considered at high risk for mpox include men who have sex with men; people who are transgender, nonbinary or gender-diverse; people who have HIV; people eligible to take medicine that reduces the risk of getting HIV; those traveling to areas of the world with a Clade I mpox outbreak who anticipate sexual contact while traveling; people planning to attend a commercial sex event; or those who have a sex partner with the above risks.

Vaccination is also urged for people who have an occupational risk for exposure, such as certain lab workers.

Telltale symptoms of mpox “include rash or unusual sores that look like pimples or pus-filled blisters on the face, body and genitals, fever, chills, headache, muscle aches or swelling of lymph nodes,” according to the L.A. County Department of Public Health.

Mpox is unlikely to spread through casual contact, or in close quarters like on a plane or in an office or store, state health officials say.

People who have symptoms of mpox should contact their healthcare provider promptly and ask to get tested. People exposed to mpox should get vaccinated as soon as possible and before symptoms develop, state health officials say. Those who have mpox should stay at home and avoid contact with other people until their rash is healed.