U.S. Confirms First Case of More Severe Mpox Strain

A patient in California tested positive for a new version of mpox, the disease formerly called monkeypox, the Centers for Disease Control and Prevention (CDC) reported last week. It’s the first confirmed case of the viral variant in the United States.

There are two distinct genetic types of the mpox virus—clade I and clade II. Clade II was responsible for the 2022 mpox outbreak in the U.S., and it’s still in circulation, with about 3,000 cases reported nationwide this year.

Clade I mpox is mostly circulating in Central and East Africa, where more than 11,000 cases have been confirmed this year. In August, the World Health Organization declared a global health emergency in response to the mpox outbreak in the Democratic Republic of the Congo and other African countries.

The California patient had recently traveled to East Africa, the CDC said. Their symptoms are reportedly improving.

“We’re only a plane ride away from any infectious disease, and given that this has been present for months, it’s only a matter of time that we see occasional cases transported to other parts of the world,” said David Weber, MD, MPH, a professor and medical director of the Department of Infection Prevention at the University of North Carolina Medical Center.

Clade I has historically been more likely to cause severe illness and death than clade II. In earlier outbreaks, the mortality rate for clade I was between 3% and 11%, compared to about 1% to 4% for clade II. In the current outbreak, the clade I death rate is closer to 1%.

The CDC said the most recent cases of sub-clade Ib have been reported in Germany, India, Kenya, Sweden, Thailand, Zimbabwe, and the United Kingdom.

How Transmissible Is Clade I Mpox?

As there is only one confirmed case of clade I mpox in the U.S., health officials can’t be sure exactly how it will spread in communities here. The CDC says the groups at highest risk for infection include men who have sex with men, gender-diverse individuals, sex workers, and some healthcare and laboratory workers.

Mpox certainly spreads through skin-to-skin contact during sex. It’s not entirely clear if the mpox virus can transmit through bodily fluids during sex. In the Democratic Republic of Congo, which is seeing the largest clade I outbreak, the virus appears to spread mostly through heterosexual contact.

Notably, two-thirds of the mpox cases in DRC are in children younger than 15 years. However, the CDC said it’s unlikely that the U.S. will see a similarly high number of cases in children. People in the U.S. tend to live in households with fewer people, have greater access to disinfecting products, and have better access to medical care and health information.

“We’re learning as we go along,” Kelly Gebo, MD, MPH, professor of medicine at the Johns Hopkins School of Medicine, told Verywell.

“The modeling outbreaks have shown that close contact transmission within and between households is unlikely in the U.S. It’s possible. But I think, in general, the way that people tend to live in the U.S. tends to be different than what has been seen in Central and East Africa, and the risk to most Americans is very low,” Gebo added.

Gebo said that with the arrival of the new viral variant, providers should be on the lookout for signs of mpox in children and women, as well as in men and gender-diverse people.

Weber said that to keep tabs on the virus, it’s important for health systems to have better access to rapid testing than they currently do. Most hospitals cannot test for mpox in-house—they must send samples to their state health department, which may send them to a different lab. That could delay providers’ ability to treat their patients and hamper surveillance efforts.

“There are other diseases that can look like mpox. We need to know if it’s mpox or syphilis or herpes or chicken pox, so we need to have easy accessibility to tests,” Weber said.

How Can You Reduce Your Risk of Mpox Infection?

The risk of mpox infection in the U.S. is very low. Still, certain groups are at higher risk for infection, especially by clade II mpox.

Mpox commonly appears as a rash. Lesions can appear on the hands, chest, and genitals. Though less common in women, mpox has appeared in transgender populations.

Gebo said that if someone in your household has mpox or has been exposed to it, you should avoid touching their sheets and other household products. Don’t share toothbrushes, sex toys, or other materials that may have bodily fluids on them.

The CDC recommends that certain high-risk groups get vaccinated with Jynneos, the only vaccine that is approved to treat mpox. It’s a two-dose series, given at least 28 days apart. People who have recovered from mpox do not need to be vaccinated, and the CDC does not recommend getting a “booster” for people who have already received two shots.

“There’s no reason to think [the Jynneos vaccine] would not be equally good against all the current clades, and there is evidence that the immunity is more durable than we think, so it’s lasting at least two years,” Weber said.

The only available antiviral treatment for mpox is called Tpoxx (tecovirimat). A study sponsored by the National Institute of Allergy and Infectious Diseases indicates that people infected with clade I mpox in the DRC who received tecovirimat did not recover any faster than those who didn’t take the treatment.

There are no completed randomized controlled trials testing how well Tpoxx works in people in the U.S. However, a study of people who took Tpoxx between 2022 and 2023 found that the medication aided in recovery and reduced the severity of the disease for most people.

Weber recommends that anyone who travels abroad read the CDC’s travel pages to learn how to protect themselves from mpox and other infectious diseases.

“Healthcare providers need to remember that we are only a plane ride away from any disease in the world,” Weber said. “Every hospital needs to be aware of the highly contagious diseases and be prepared to both diagnose and treat them.”