SF Department of Public Health confirms first clade I mpox case

April 18, 2026 - 02:50

The San Francisco Department of Public Health has announced the city's first confirmed case of a distinct and potentially more severe strain of the mpox virus, known as clade I. The case was identified in a resident who recently traveled to a country where this clade is endemic. Health officials stress that the overall risk to the public remains low, but this development marks a significant shift in the local mpox landscape.

Previously, all cases in San Francisco and across the United States during the 2022 outbreak were caused by the clade IIb strain, which generally causes less severe illness. The newly detected clade I strain, historically endemic in parts of Central Africa, is associated with higher rates of severe disease and mortality. The infected individual is currently isolating and receiving care, and close contacts have been notified.

The health department is urging vigilance, particularly among individuals who have recently traveled to endemic regions or who have specific risk factors. Vaccination remains the most effective tool for prevention. Officials emphasize that the two-dose Jynneos vaccine, widely available in the city, is expected to provide strong protection against severe illness from both clade I and clade II mpox. Health care providers have been alerted to be aware of the symptoms, which include a characteristic rash and fever, and to test suspected cases promptly.