The first U.S. case of a potentially more severe type of mpox (formerly monkeypox), known as Clade I, has been reported in San Mateo, south of San Francisco, the California Department of Public Health (CA DPH) announced on November 16. Health officials consider the risk to the general public to be low, but they are urging people at higher risk—including gay men and transgender people—to get vaccinated.

“Given the very low risk to the public of exposure to Clade I, the standing guidance for preventing mpox has not changed,” said San Mateo County Health Officer Kismet Baldwin-Santana, MD.

The California 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 (CDC) for further viral characterization.

The affected person is isolating at home and recovering, according to the CA DPH news release. “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.”

Clade I and Clade II Mpox

There are two major strains of mpox, Clade I and Clade II. The latter type 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.

Concurrent Clade I mpox outbreaks are underway in the Democratic Republic of the Congo (DRC) 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, including Sweden and Thailand, and a small cluster of cases attributed to household contact was identified in the United Kingdom in early November. The California patient is the first Clade I case reported in the U.S.; the CA DPH announcement did not indicate whether this person has subclade 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 0.2%, but people with advanced HIV are more likely to develop severe illness, and most people who died were Black gay men with AIDS.

Fatality estimates for Clade I mpox in Africa have ranged up to 10%, but mortality depends on access to medical care. A recent trial of the antiviral drug TPOXX (tecovirimat) in the DRC found that although the drug did not appear to be effective against Clade I mpox, the mortality rate among study participants—whether they received TPOXX or not—was about half the rate for all cases in the DRC (1.7% versus 3.6%), indicating that better outcomes can be achieved with good supportive care.


“Death rates [for Clade I mpox] 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 CDC notes that there have been no deaths linked to travel-associated subtype Ib mpox outside of Africa, and available data shows “relatively mild disease courses.”

Mpox Vaccine

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. The virus does not appear to spread via airborne transmission without close contact.

“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 mpox outbreaks as “low” for the overall U.S. population and “low to moderate” for gay and bisexual men and people in their sexual networks. Federal experts think transmission of Clade I 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 absence of animal reservoirs.

Federal, state and local health officials are urging people at higher risk to get vaccinated against mpox regardless of whether 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 others in their sexual networks. Evidence indicates that the vaccine protects against both Clade I and Clade II 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 dose 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. A CDC study found that only around a quarter of eligible individuals have received both doses of the vaccine, and most of the recent Clade II mpox cases in the U.S. have involved people who are not fully vaccinated.

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.

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