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.

Past Updates: WHO declares highest alert over monkeypox

The monkeypox outbreak has been declared a global health emergency by the World Health Organization.

The classification is the highest alert that the WHO can issue and follows a worldwide upsurge in cases.

It came at the end of the second meeting of the WHO’s emergency committee on the virus.

More than 16,000 cases have now been reported from 75 countries, said WHO director general Dr Tedros Adhanom Ghebreyesus.

There had been five deaths so far as a result of the outbreak, he added.

There are only two other such health emergencies at present – the coronavirus pandemic and the continuing effort to eradicate polio.

Monkeypox outbreak was first said to only be spread by sex

European nations have confirmed dozens of cases in what’s become the largest outbreak of monkeypox ever on the Continent, according to the German military. The U.S. and Canada each have at least five confirmed or presumed cases so far. Belgium just introduced a mandatory 21-day quarantine for monkeypox patients.

The WHO convened an emergency meeting this weekend via video conference to look at the virus, identify those most at risk and study its transmission. The organization will hold a second global meeting on monkeypox next week to more thoroughly study the risks and treatments available to fight the virus.

While the virus itself is not a sexually transmitted infection, which are generally spread through semen and vaginal fluids, the most recent surge in cases appears to have been spread among men who have sex with other men, WHO officials said, emphasizing that anyone can contract monkeypox.

“Many diseases can be spread through sexual contact. You could get a cough or a cold through sexual contact, but it doesn’t mean that it’s a sexually transmitted disease,” said Andy Seale, who advises the WHO on HIV, hepatitis and other sexually transmitted infections.

Spanish health authorities believe that a string of new monkeypox cases is linked to a gay sauna near Madrid and a Pride event in the Canary Islands that drew tens of thousands of people. Spain announced 23 new cases Friday. Madrid regional health chief Enrique Ruiz Escudero told journalists that health officials have been tracing the cases from an outbreak at the now-closed sauna, Reuters reports.

“The Public Health Department will carry out an even more detailed analysis… to control contagion, cut the chains of transmission and try to mitigate the transmission of this virus as much as possible,” Escudero said.

The virus is spread through close contact with people, animals or material infected with the virus. It enters the body through broken skin, the respiratory tract, the eyes, nose and mouth. Though human-to-human transmission is believed to occur through respiratory droplets as well, that method requires prolonged face-to-face contact because the droplets cannot travel more than a few feet, according to the CDC.

Monkeypox is a zoonotic orthopoxvirus that appears similar to smallpox, although significantly less deadly. Most outbreaks in Europe and elsewhere in the Western Hemisphere are related to the exotic pet trade and international travel.

Early symptoms of monkeypox include a fever, headache, back pain, muscle aches and low energy, WHO officials said. That then progresses to a rash on the face, hands, feet, eyes, mouth or genitals that turns into raised bumps, or papules, that then become blisters that often resemble chicken pox. Those can then fill with a white fluid, becoming a pustule, that breaks and scabs over. Doctors describe it as a disabling disease that can last two to four months and has a lengthy 21-day incubation period.

He said to expect more confirmed cases in the U.S. in the coming weeks as doctors and public health officials reevaluate patients who have presented with symptoms and the virus continues to spread.

Monkeypox is a disease caused by a virus in the same family as smallpox but is not as severe, according to the Centers for Disease Control and Prevention. However, monkeypox can kill as many as 1 in 10 people who contract the disease, based on observations in Africa, according to the CDC.

The vaccine used to prevent smallpox appears to be about 85% effective in guarding against monkeypox in observational research in Africa, WHO officials said. But the vaccines aren’t widely available so it’s important to reserve them for populations that are most at risk, said Maria Van Kerkhove, the WHO’s leading epidemiologist on zoonotic diseases. She said the WHO will be teaming up with vaccine makers to see if they can ramp up production.