Rare Mpox Strain Detected In San Francisco
- Health officials report a surge of mpox cases and a rare strain detected in San Francisco. - Officials urge vaccination after the rare strain appears locally for the first time. - Rising case counts could prompt increased public-health outreach and expanded vaccine clinic efforts (patch.com).
Mpox is a virus that usually spreads through close skin-to-skin contact, and San Francisco has now confirmed its first local case of the rarer clade I strain. (sf.gov) San Francisco health officials said April 16 that the case was confirmed on April 14 in an unvaccinated adult who was hospitalized and is improving. The patient reported close contact with someone who had traveled internationally. (sf.gov) There are two main versions of the virus, called clades. The 2022 U.S. outbreak was driven by clade II, while clade I has been tied to outbreaks in eastern and central Africa and has only rarely been identified in the United States. (sf.gov) (kqed.org) California health officials said this was the seventh identified clade I case in the state since November 2024 and the first in San Francisco. They also said public-health teams were doing enhanced surveillance and contact tracing to look for additional cases. (cdph.ca.gov) The state’s warning came as ordinary clade II mpox cases were already climbing. California said it is averaging 14.5 weekly clade II cases in 2026, up from 5.8 in 2024 and 3.4 in 2025. (cdph.ca.gov) San Francisco saw the same shift earlier in the year. The city said 24 residents were diagnosed with clade II mpox from January through March 2026, after fewer than 10 cases in the first quarter of prior years. (sf.gov) Federal data shows most U.S. mpox cases are still clade II and continue to occur mainly in people who were unvaccinated or had received only one JYNNEOS dose. The Centers for Disease Control and Prevention says two doses provide maximum protection. (cdc.gov) San Francisco and California officials said the vaccine protects against both clade I and clade II. They said the current risk to the general public remains low, but urged people at higher risk to complete the two-dose series before summer travel and large events. (sf.gov) (cdph.ca.gov) Health officials say mpox symptoms can begin with fever, swollen lymph nodes or fatigue, followed by a rash that looks like pimples or blisters. People with a new rash are being told to get tested and notify close contacts quickly. (sf.gov) For now, San Francisco’s first clade I case has not changed the city’s core message: the virus is still spreading mostly through close contact, and the main tool officials are pushing is vaccination. (sf.gov)
