Rare Mpox Strain Detected In San Francisco

San Francisco has confirmed its first known clade I mpox case, a rarer strain that health officials say can cause more severe illness than the virus behind the 2022 outbreak. (sf.gov) The San Francisco Department of Public Health said April 16 that the case was identified in a city resident on April 14. The patient was an unvaccinated adult, was hospitalized, and is improving. (sf.gov) Officials said the San Francisco patient reported close contact with someone who had traveled internationally, and the city described the case as travel-linked rather than evidence of broader local spread. State officials said the risk to the general public remains low. (sf.gov) (cdph.ca.gov) Mpox is a viral disease that usually spreads through close, often skin-to-skin contact. The strain that drove the 2022 U.S. outbreak was clade II; clade I is a different branch of the virus that has been tied to outbreaks in Central and Eastern Africa and has appeared only rarely in the United States. (cdc.gov) (sf.gov) California’s warning came as overall mpox activity was already rising. The California Department of Public Health said weekly clade II case averages in 2026 were running at 14.5 cases, up from 5.8 in 2024 and 3.4 in 2025. (cdph.ca.gov) San Francisco has seen the same pattern. City health officials said 24 residents were diagnosed with clade II mpox from January through March 2026, compared with fewer than 10 cases in the first quarter of prior years. (sf.gov) State and local agencies are pushing vaccination because most recent California cases have been in people who were not vaccinated or had received only one dose. California says the two-dose JYNNEOS vaccine protects against both clade I and clade II mpox. (cdph.ca.gov) (cdc.gov) The Centers for Disease Control and Prevention says the second JYNNEOS dose should be given four weeks after the first, though people should still get dose two even if they are late. California and local health departments are urging vaccination ahead of summer travel and large events, when close-contact transmission has risen before. (cdc.gov) (sf.gov) Federal and state officials have counted only a small number of clade I cases in the United States so far, including six in California, according to San Francisco’s health alert. For now, the city’s message is narrow: people at higher risk should get both shots before the next seasonal uptick. (sf.gov)