The first U.S. case of a potentially more severe variant of mpox, known as clade I, has been reported in San Mateo County, the California Department of Public Health announced Saturday, November 16. Health officials consider the risk to the public to be low, but they are urging people at higher risk — including gay and bisexual men — to get vaccinated.
San Francisco public health officials are also monitoring the situation. "At this time, there are no reported cases of clade I in San Francisco," the department stated over the weekend.
Both San Francisco and San Mateo health officials said the risk of clade I is low.
"Given the very low risk to the public of exposure to clade I, the standing guidance for preventing mpox has not changed," San Mateo County Health Officer Dr. Kismet Baldwin-Santana stated.
The San Mateo patient had recently traveled from eastern Africa, where there is an ongoing clade I mpox outbreak. The individual had mild illness and was treated at a local medical facility shortly after returning to the United States. Laboratory testing confirmed that the person had clade I, and specimens are being sent to the Centers for Disease Control and Prevention for further viral characterization.
"The affected individual received health care in San Mateo County based on their travel history and symptoms," according to the CA DPH release. "The individual is isolating at home and recovering. People who had close contact with this individual are being contacted by public health workers, but there is no concern or evidence that mpox clade I is currently spreading between individuals in California or the United States."
Two types of mpox
There are two major strains of mpox, clade I and clade II. Clade II was responsible for the global outbreak in 2022, which mainly affected gay and bisexual men. This strain continues to circulate at a low level in the U.S. When the CDC stopped updating its national count this past January, it had tallied more than 32,000 total cases, resulting in 58 deaths. In San Francisco, 31 cases have been reported so far this year, according to the San Francisco Department of Public Health website.
Concurrent clade I mpox outbreaks are underway in the Democratic Republic of the Congo and nearby countries, prompting the World Health Organization to declare a public health emergency of international concern in August. More than 12,000 confirmed cases and 53 confirmed deaths have been reported in Africa in 2024, but testing is limited, and suspected cases exceed 50,000.
One outbreak in central and east Africa, involving viral subtype Ia, mainly affects children and is primarily spreading via close physical contact and contact with wild animals. The other, involving subtype Ib, appears to be largely driven by sexual transmission — both heterosexual and homosexual — and has affected many sex workers.
Isolated cases of clade I mpox have been reported in several other countries outside of Africa in recent months, and a small cluster of cases attributed to household contact was identified in the United Kingdom in early November. The San Mateo patient is the first clade I case reported in the U.S. The CA DPH announcement did not indicate whether this person has clade Ia or Ib.
Clade I mpox has historically had a higher fatality rate than clade II, especially among children. During the 2022 clade II outbreak in the U.S., the mortality rate was around 0.2%, but people with advanced HIV are more likely to develop severe illness. Fatality estimates for clade I mpox in Africa have ranged up to 10%, but mortality depends on access to medical care.
"Death rates are expected to be much lower in countries with stronger healthcare systems and treatment options, including the United States," according to a CDC news release. "Current data supports that subclade Ib has a lower death rate of less than 1% both in and outside of Africa. The recent travel-associated clade I mpox cases outside of Africa have all been attributed to subclade Ib; there have been no deaths associated with these cases and available data for a subset has detailed relatively mild disease courses."
Mpox vaccination urged
Mpox is primarily transmitted via skin-to-skin contact, including sex, hands-on caregiving, and contact between members of a household. It can also spread through saliva, respiratory droplets at close range and contact with materials such as clothes or bedding used by people with mpox lesions.
"Casual contact, like one might have during travel, in an office, classroom, or store, is unlikely to pose significant risks for transmission of mpox," according to the CA DPH announcement.
As of November 13, the CDC assesses the risk from the central and east Africa outbreaks as "low" for the overall U.S. population and "low to moderate" for gay and bisexual men and people in their sexual networks. Although a small number of travel-associated clade I mpox cases are expected, federal experts think transmission is likely to be low compared with African countries due to smaller and less crowded households, better access to improved sanitation and health care, and the lack of animal reservoirs.
Mpox causes a painful rash that can appear anywhere on the body, sometimes accompanied by fever and other flu-like symptoms. Standard treatment involves supportive care and medications, though a recent study found that the antiviral drug TPOXX (tecovirimat) is not as effective for clade I mpox.
Federal, state, and local health officials are urging people at higher risk to get vaccinated regardless of whether or not a local outbreak is underway. These include men who have sex with men, transgender and nonbinary individuals, people living with HIV, those using or eligible for PrEP, sex workers and those in their sexual networks. There is currently no vaccine shortage, and SF DPH offers it to anyone who wants protection from mpox.
Two doses of the Jynneos vaccine should be given at least four weeks apart. People who received only one dose in 2022 can get the second at any time and do not need to restart the series. Booster doses are not recommended for those who have completed the two-dose series, and people who have already had mpox do not need to be vaccinated. Evidence indicates that the vaccine protects against both clade I and II mpox. A CDC study found that only around a quarter of eligible individuals have received two doses of the vaccine nationwide, but the rate is considerably higher in San Francisco.
"SF DPH along with Bay Area, state and federal partners continue to monitor clade I mpox in the United States," according to a statement the health department sent to the Bay Area Reporter. "The two-dose mpox vaccine provides the best protection against mpox, including clade I. ... The mpox vaccine is available through health systems and clinics. Those who do not have insurance or are having difficulty accessing care are welcome to visit SF DPH's San Francisco City Clinic. We will continue to update the community if further actions are needed to protect health."
While mpox testing, vaccines, and treatment are now readily available in the U.S., this is not the case in Africa. Advocates stress that providing these tools is critical to limit the spread of the disease both within Africa and to other countries.
For more information on mpox vaccines, testing, and treatment, go to sf.gov/get-mpox-vaccines-testing-and-treatment.
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