Rare Mpox Strain Detected In San Francisco
- Health officials report a surge in mpox cases statewide and a rare strain has been identified in San Francisco. - Officials are urging Californians to get vaccinated as the rare strain appears for the first time locally. - Public-health response includes increased testing and vaccination outreach to limit transmission and monitor variants (patch.com).
San Francisco has identified its first clade I mpox case, a rarer strain that California health officials say can cause more severe illness. (sf.gov) The San Francisco Department of Public Health said April 16 that the case was confirmed 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) California’s health department said this was the seventh identified clade I case in the state since November 2024 and the first in San Francisco. Officials said they are doing enhanced surveillance and contact tracing to find additional cases. (cdph.ca.gov) Mpox is a viral disease that usually spreads through close, often skin-to-skin contact, and the strain that drove the 2022 outbreak in the United States was clade II. Clade I is a different branch of the virus that has been tied to a large outbreak in eastern and central Africa since 2023. (cdc.gov) Federal health officials said the risk to most people in the United States remains low, but clade I has now appeared in scattered U.S. cases beyond travel-linked infections. The Centers for Disease Control and Prevention said 11 U.S. clade I cases were reported from November 2024 through February 2026, with five more reported since March 2026. (cdc.gov) The San Francisco alert landed as clade II mpox cases were already rising locally. City health officials said 24 San Francisco residents were diagnosed from January through March 2026, compared with fewer than 10 cases in the first quarter of prior years. (sf.gov) State officials also said California’s weekly confirmed clade II cases have climbed in 2026, averaging 14.5 cases a week, up from 5.8 in 2024 and 3.4 in 2025. Most recent California infections have been in unvaccinated people, according to the state advisory. (cdph.ca.gov) Health officials are urging people at higher risk to get both doses of the JYNNEOS vaccine, which San Francisco and California say protects against both clade I and clade II. The state said the timing matters with summer travel and large events approaching. (sf.gov)
