Three cases of Clade I mpox (also known as monkeypox) have been identified in Southern California, state health officials announced in mid-October. The three apparently unrelated cases were reported in long Beach and Los Angeles. None of the three people were previously vaccinated against mpox; all were hospitalized and are recovering at home.
None of these people had recently traveled outside the United States, suggesting that local person-to-person community spread is likely occurring in California, “primarily impacting communities of gay and bisexual men, and other men who have sex with men, as well as their social networks,” according to the California Department of Public Health.
The European Centre for Disease Prevention and Control also recently reported the first cases of locally acquired Clade I mpox in Spain, the Netherlands, Italy and Portugal. Two of the three patients with available information were men who have sex with men
The first case of Clade I mpox in the U.S. was reported in San Mateo, south of San Francisco, in November 2024. This individual, who developed mild illness, had recently traveled from East Africa. About half a dozen other U.S. Clade I cases also involved people who had recently traveled from countries experiencing mpox outbreaks.
California health officials consider the risk to the general public to be low, but they urge people at higher risk—including gay men and gender-diverse people—to get the mpox vaccine if they are not already immune.
“As we continue to monitor the situation, it is crucial for Californians to stay informed and take preventive measures, especially persons who are more likely to be exposed to mpox,” state assistant health officer Rita Nguyen, MD, said in the CDPH news release. “Risk of severe disease and hospitalization are highest for people with weakened immune systems, so it’s critical to protect yourself by getting both doses of the mpox vaccine if you or your sex partner(s) may be at risk for mpox.”
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 Centers for Disease Control and Prevention (CDC) stopped updating its national count in January 2024, it had tallied more than 32,000 total cases, resulting in 58 deaths.
Several outbreaks of Clade I mpox are ongoing in Central and East Africa, especially the Democratic Republic of Congo and nearby countries. Since January 1, 2024, more than 46,000 laboratory-confirmed cases and over 200 deaths have been reported, according to the CDC. Some of the concurrent outbreaks involve viral mpox subtype Ia, which historically has mainly affected children and is primarily spread via close physical contact and contact with wild animals, while others are due to subtype Ib, which appears to be largely driven by sexual transmission, both heterosexual and homosexual.
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 just 0.2%, but people with advanced HIV or AIDS were more likely to develop severe illness. Fatality estimates for Clade I mpox in Africa have ranged up to 10%, but malnutrition and inadequate health services contribute to high mortality, and it’s not clear whether this type is inherently more likely to cause severe illness and death.
Protect Yourself
Mpox is primarily transmitted via skin-to-skin contact, including sex, hands-on caregiving and contact between members of a household. It can also potentially spread through saliva, respiratory droplets at close range and contact with materials such as clothes or bedding used by a person with mpox lesions. The virus does not appear to spread via airborne transmission without close contact.
People who experience mpox symptoms should seek care and testing. The characteristic sign is a rash or sores anywhere on the body—including the face, genitals, anal area, palms or soles of the feet—sometimes accompanied by fever, swollen lymph nodes and other flu-like symptoms. People with severe cases have been treated with TPOXX (tecovirimat), but two recent clinical trials found that the antiviral is not very effective against either Clade I or Clade II mpox.
Health officials urge people at higher risk to get vaccinated against mpox whether or not a local outbreak is currently underway. These include men who have sex with men, transgender, nonbinary and other gender-diverse individuals, people living with HIV, those using or eligible for HIV pre-exposure prophylaxis, sex workers and those in their sexual networks.
Evidence indicates that the Jynneos MVA-BN vaccine protects against both Clade I and Clade II mpox. People who were not vaccinated during or since the 2022 outbreak should receive two doses spaces at least a month apart. People who received a single dose during that outbreak can get their second dose at any time. Those who were recently exposed to mpox can still benefit from vaccination within several days. Those who have previously had mpox are considered to be immune.
Vaccine boosters currently are not recommended for people who have already received both Jynneos doses. Many of the people who have contracted Clade II mpox since the 2022 outbreak were fully vaccinated—so the vaccine does not confer complete protection—but their symptoms were generally mild.
Unfortunately, the latest mpox case reports come at a time when federal funding cuts, CDC staff layoffs and the government shutdown could compromise an efficient response.
“The infrastructure we built during the 2022 outbreak has just been eviscerated,” Joseph Osmundson, PhD, of New York University, told the New York Times. “The very things we need to understand if we have a problem now, and if we will have a problem in the future, are being systematically dismantled.”
Click here for more news about mpox.
