Health officials in Chicago have reported a new cluster of 13 mpox (formerly monkeypox) cases, following a small outbreak in France in March. Although the number of cases remains low, the new clusters raise concern about the possibility of a resurgence this summer.

Experts are urging people at risk—primarily sexually active gay men—to get both doses of the Jynneos mpox vaccine. Vaccination is especially important for people with advanced HIV, who are most likely to develop severe mpox illness. However, more than half of people in the recent clusters were fully vaccinated, suggesting that immunity may wane after vaccination or prior infection.

“Without renewed vaccination and prevention efforts, we are at risk for a resurgence of mpox,” White House mpox deputy coordinator Demetre Daskalakis, MD, MPH, told NBC News. “The vaccine is a really important tool, even if it’s not perfect.”

Mpox cases have declined dramatically since the outbreak peaked late last summer, likely due to a combination of behavior change, vaccination and natural immunity after infection. In late April, the Centers for Disease Control and Prevention (CDC) reported the first week with zero new cases. To date, the CDC has identified 30,361 mpox cases in the United States and more than 87,000 cases worldwide. The overwhelming majority have been among gay men.

As of May 9, Howard Brown Health in Chicago, which serves the LGBT community, has diagnosed eight new mpox cases since mid-April, after diagnosing only one case during the preceding three months. Chicago health officials have identified a total of 12 confirmed cases and one probable case. The new cluster is the largest seen in the United States so far this year. As was the case during the summer surge, the newly diagnosed individuals are gay or bisexual men. Of note, nine (69%) were fully vaccinated. All cases were mild, and none required treatment, suggesting that the vaccine prevents severe illness even if it’s not fully protective against infection.

Like last summer’s outbreak, the case trajectory in the United States may be following Europe’s lead. In late March, French health officials reported 18 new mpox cases since the prior report in January, 14 of them since March 1. Most were men who have sex with men, and all but one were diagnosed in the Centre-Val de Loire region south of Paris. Five of the men had received two doses of the Jynneos vaccine in 2022, and another five were vaccinated against smallpox as children and received one vaccine dose last year. (The Jynneos vaccine, known as Imvanex in Europe, protects against both smallpox and mpox.)

Outside of France, a May 4 update from the European Centre for Disease Prevention and Control and the World Health Organization European Region reported 17 new cases during the past four weeks, including five in Spain and three in the United Kingdom. The French cluster in March was included in the previous monthly report; only two new cases were diagnosed in France in April, indicating that there does not appear to be an ongoing outbreak.

Although the number of mpox cases worldwide continues to decline overall, there are other trouble spots, according to the World Health Organization (WHO). WHO’s African Region reported a 7% rise in mpox cases during the last two weeks of April; of the 111 newly confirmed cases, 106 were in the Democratic Republic of Congo. Japan and South Korea have also seen recent outbreaks.

Given the bump in new cases, health officials are reemphasizing the importance of getting two doses of the vaccine. Gay and bisexual men, transgender men and women, and others in their sexual networks lined up for vaccines last summer, but many only received their first dose. According to the CDC, less than a quarter of people at risk are fully vaccinated and just 37% have received one dose. White men are more likely to be vaccinated than Black and Latino men, and some groups still have low vaccination rates, including people who do not self-identify as gay or bisexual, those living in the South and outside urban centers and those with limited access to health care.

People with advanced HIV and a low CD4 count are at greatest risk for severe mpox. Of the 42 people who have died with mpox in the United States, most were Black gay men with advanced HIV. Along with vaccination, starting and staying on antiretroviral treatment is the best way to reduce the risk for mpox complications.

“We urge sexually active members of our community to receive the mpox vaccine. For example, unvaccinated people planning to attend International Mr. Leather at the end of May should receive their first dose of the mpox vaccination as soon as possible,” Howard Brown Health chief medical director Patrick Gibbons, DO, said in a press release. “The more people who get vaccinated, the better protected the LGBTQ+ community will be from another outbreak of monkeypox this year.”

“We want to make sure that everyone can enjoy a happy and healthy Pride,” said San Francisco health officer Susan Philip, MD, MPH. “If you received your first dose of the mpox vaccine, even if it was in the fall, it is not too late—now is a great time to get your second dose.” People who get their first dose now will be eligible for a second dose four weeks later and should be fully protected by Pride events in late June.


Editor’s note: This article has been updated with new numbers from the Chicago Department of Public Health.

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