Health Experts are Watching a More Dangerous Version of Mpox

Cases of a new group of mpox viruses in the Democratic Republic of Congo have U.S. health officials on alert.

Cases of a new group of mpox viruses are rising, potentially posing a risk to people around the world, according to health officials at the U.S. Centers for Disease Control and Prevention (CDC).

So far, the cases have been centered in the Democratic Republic of Congo (DRC) and have not spread beyond Central African countries where the virus remains endemic, the CDC wrote in a report on May 16. But health officials are concerned because this group of viruses—known as clade I mpox viruses—is known to cause more severe illness than the clade II viruses responsible for the previous mpox outbreaks in 2022, which originated in Nigeria. Clade I mpox viruses have a higher fatality rate—killing anywhere from 1.4% to more than 10% of infected people—than clade II, which has a 0.1% to 3.6% mortality rate.

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The 2022 outbreaks spread primarily through sexual contact among men who have sex with men. An effective, two-dose vaccine helped to protect these higher risk groups and kept the infections from expanding into an epidemic. (The current vaccine also works against the latest clade I viruses.)

But with a recent increase in global travel, health officials are watching for spread of the latest clade I viruses outside of the regions where it is endemic. According to the CDC report, health officials in DRC reported that clade I infections were reported in 25 of 26 provinces of the country, and that young people were the hardest hit: 67% of cases and 78% of deaths occurred in people 15 years or younger. The virus can cause more severe disease in those with weakened or compromised immune systems.

Researchers believe that the infections were caused by exposure to infected animals, which then spread quickly from person to person through close contact in households.

Read More: What It Really Feels Like to Have Mpox

In the U.S., the CDC recommends that doctors test anyone suspected of having mpox, either because of recent travel to affected areas in Africa, contact with someone who has recently been to Africa, or exposure to someone with mpox. Symptoms include fever, headache, rash, and painful sores. Existing tests for mpox are designed to pick up orthopoxvirus, the group of viruses to which mpox (and smallpox) belong, but these tests cannot detect whether an infection belongs to clade I or II. Some tests available at specific laboratories can detect clade II viruses, and therefore rule out clade I mpox, but to be certain, CDC is conducting further genetic analysis on positive orthopox samples sent from U.S. labs to the agency to determine which type of virus it is.

No clade I cases have yet been found in the U.S. According to the CDC, 343 samples tested positive for orthopox from December 2023 to mid April 2024, and further testing showed no clade I viruses. To better capture any potential infections among people who might not get tested, the agency also began looking for mpox last December in wastewater, and so far, positive signals correspond to areas from which clade II viruses were found. Samples collected at four airports through the CDC’s Traveler-Based Genomic Surveillance program from passengers who volunteer to provide samples did not detect any orthopox virus since last December.

For now, the CDC is working with health officials in DRC to track and respond to the current outbreak there. The effective vaccines that most of the world turned to in response to the 2022 outbreaks are not authorized in DRC, which increases the risk that cases will continue to rise and potentially spread beyond that region of the world.

In the U.S., the agency is urging doctors to educate their at-risk patients, which include gay and bisexual men and men who have sex with men, to get the vaccine. Since the 2022 outbreaks, only 23% of people at risk of mpox infection in the U.S. have received two doses. “Collaboration among global health partners is now urgently needed to assist DRC in procuring and delivering sufficient vaccine where it is most needed,” the CDC scientists write in the report.