More Severe Mpox Strain, Clade I, Spreads in California

Health officials announced today that three California residents have been infected with clade I mpox, a more severe strain of the virus, marking the first instance of local spread of this type within the United States.

The unrelated cases, identified in Long Beach and Los Angeles County, involve patients who had not recently traveled internationally. All three individuals required hospitalization but are currently recovering, according to the California Department of Public Health. Mpox typically begins with flu-like symptoms followed by a distinctive rash. This development raises concerns as clade I has historically caused more severe illness and higher mortality rates in Central and East Africa, particularly in the Democratic Republic of the Congo.

“The presence of clade I needs careful evaluation,” said Seth Blumberg, an associate professor of medicine at the University of California, San Francisco’s division of infectious diseases and hospital medicine. “I don’t think the historical results we’ve seen from Africa are directly translatable, but we need to monitor these cases closely.” Dr. Rita Nguyen, assistant state public health officer, emphasized that “clade I mpox cases can be severe” and that the “risk of severe disease and hospitalization are highest for people with weakened immune systems.” The Centers for Disease Control and Prevention has more information on mpox and prevention.

While the overall risk to the public remains low, California’s public health department believes person-to-person spread of clade I is occurring, primarily within gay and bisexual men and transgender or non-binary people who have sex with men. The Jynneos vaccine, which protects against both clade I and II mpox, is available and recommended, especially for those at higher risk; you can find vaccination locations through your MyTurn account. The emergence of this strain underscores the importance of continued vigilance and vaccination efforts to prevent a wider outbreak, building on lessons learned from the 2022 outbreak.

Officials stated they are continuing to analyze the cases and monitor for any further spread, and encourage anyone who suspects they may have mpox to seek medical attention.