Rare Mpox Strain Detected In San Francisco
- Health officials report mpox cases rising across California, with a rare strain detected for the first time in San Francisco. - Officials are urging Californians to get vaccinated as public health agencies monitor spread and potential exposures. - The detection raises concern about transmission chains and vaccine demand in the Bay Area (patch.com).
Mpox is a viral infection that usually spreads through close skin-to-skin contact, and San Francisco has now confirmed its first case of clade I, the rarer strain tied to recent international spread. (sf.gov) San Francisco health officials said April 16 that the case was confirmed on April 14 in an unvaccinated adult who had close contact with someone who traveled internationally to an area where clade I mpox is circulating. The patient was hospitalized and is improving. (sf.gov) California’s public health department said April 17 that this was the seventh identified clade I case in California since November 2024 and the first in San Francisco. State officials said they are doing enhanced surveillance and contact tracing. (cdph.ca.gov) The strain behind the 2022 U.S. outbreak was clade II, not clade I. San Francisco says the 2022 outbreak has led to 1,066 cases in the city as of April 9, 2026. (sf.gov) What changed this spring is the pace of new cases. California said it is averaging 14.5 clade II cases a week so far in 2026, up from 5.8 in the same period of 2024 and 3.4 in 2025. (cdph.ca.gov) San Francisco is seeing the same pattern. The city said it had 24 clade II cases from January through March 2026, compared with fewer than 10 in the first quarter of prior years. (sf.gov) Health officials are pushing the JYNNEOS vaccine because they say it protects against both clade I and clade II. The Centers for Disease Control and Prevention says two doses are recommended for maximum protection, and most U.S. cases continue to be in people who were unvaccinated or had only one dose. (cdc.gov) San Francisco told clinicians to assess mpox risk and vaccination status at visits for men, trans, or nonbinary people who have sex with men, and for travelers expecting sex with new partners or at large events in places where clade I is circulating. The city said the two doses are given 28 days apart, and a delayed second dose can be given immediately. (sf.gov) State officials said the risk to the general public remains low, but they warned that summer travel and large events have been linked to higher transmission in past years. San Francisco issued the same warning, saying cases may rise again in summer and fall. (cdph.ca.gov; sf.gov)
