The San Mateo County Health Department said it is working with state and federal partners to respond to a reported case of Mpox Clade I (monkeypox) variant, the first known in the United States.
This first case, reported by the Centers for Disease Control and Prevention (CDC), occurred in California, was laboratory-confirmed, and is associated with an ongoing I mpox clade outbreak in central and eastern Africa and was acquired through travel.
While the risk of public exposure to clade I mpox remains very low, sporadic cases of clade II mpox continue to occur in the country.
Outbreaks from different subclades, or genetic branches, can have different characteristics, including who they affect and how they spread. Although the II mpox clade has been circulating in the United States since 2022, the I mpox clade has never been reported in the United States until now. Historically, the I mpox clade has caused more severe illness and death than the II mpox clade.
The county health system noted that it has been responding to the global mpox clade II outbreak since 2022.
The San Mateo County Health Department is assisting the California Department of Public Health and the Centers for Disease Control and Prevention in tracing and notifying close contacts of possible exposure.
To date, 108 cases of clade II mpox have been reported in San Mateo County.
“While cluster I may be new to the United States, we and other counties have been responding to mpox since 2022 with contact tracing, guidance and vaccination support,” said Dr. Kismet Baldwin-Santana, San Mateo County health director. “Our work continues, with the support of our state and federal partners.”
Health officials said the person, who has mild illness, is isolated at home and recovering, and said there is no concern or evidence that mpox clade I is currently spreading among people in San Mateo County or elsewhere in the United States.
Transmission usually occurs through close (skin-to-skin), intimate, and sexual contact. Casual contact, such as during travel, in an office, classroom, or store, is unlikely to pose significant risks for transmission of human immunodeficiency virus (MPOX). People traveling to countries where clade I outbreaks are occurring should consult with their physician.
"Given the very low risk to the public from exposure to clade I," said Dr. Baldwin-Santana, "current recommendations for preventing mpox have not changed."
People who have certain risk factors should talk with their primary health care provider about vaccination and take steps to reduce exposure, including avoiding close, skin-to-skin contact with people who have a rash that looks like MPox and avoiding contact with objects and materials that have been used by a person with MPox.
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