Clade I Mpox Emerges in the United States

San Francisco has confirmed its first case of Clade I mpox, a more virulent strain of the virus historically linked to higher fatality rates – up to 10% – compared to Clade II. The individual affected is an unvaccinated adult who had close contact with an international traveler.

Global Circulation and US Cases

This development coincides with ongoing circulation of Clade I mpox in Central and Eastern Africa, with increasing reports of locally-acquired cases in Europe and sporadic travel-related cases worldwide. Since late 2024, the United States has reported a total of 15 cases of Clade I mpox, including 6 in California. A surge in diagnoses was observed in March 2026.

San Francisco Case Details

The confirmed case in San Francisco is currently hospitalized but is reportedly showing signs of improvement. According to the Centers for Disease Control and Prevention (CDC), all U.S. cases identified to date have been connected to recent travel to regions where the virus is endemic or contact with individuals who have traveled to those areas.

Hawaii Wastewater Detection

First Detection in the State

Beyond San Francisco, the Hawai‘i Department of Health recently identified Clade I mpox in a wastewater sample collected from Joint Base Pearl Harbor Hickam on April 13, 2026. This marks the first detection of Clade I mpox in Hawaiʻi wastewater, though no clinical cases have been identified within the state as of yet.

Early Warning Signal

Officials emphasize that the detection in wastewater does not necessarily indicate widespread community transmission, but rather serves as an early warning signal. The facility where the sample was collected serves both military personnel and the public, including visitors to the Pearl Harbor National Memorial Museum.

Confirmation Timeline

The initial detection was reported on April 20, 2026, with confirmatory results received on April 24, 2026. Public health officials are actively monitoring the situation and recommending that individuals at higher risk consider vaccination with the JYNNEOS (mpox) vaccine.

Public Health Response and Prevention

The emergence of Clade I mpox in the U.S., as evidenced by cases in major cities and wastewater detections, necessitates increased vigilance. While public health agencies are striving to maintain calm, transparent reporting and proactive measures like vaccination are crucial to prevent a potential escalation.

Historical Context

The mpox outbreak that began in 2022 in the U.S. was caused by Clade II, resulting in 1066 cases in San Francisco as of April 9, 2026. The distinction between Clade I and Clade II is significant due to the higher severity associated with Clade I.

The CDC and local health departments are working to determine if Clade I mpox poses a greater risk to the U.S. population. Basic preventative measures, such as practicing safe online habits and avoiding risky behaviors, remain essential in protecting oneself from infectious diseases.