A recent uptick in mpox diagnoses in Chicago, some of them in people vaccinated against the virus, has raised concerns about a possible increase in cases among gay and bisexual men during the summer. 

On Monday, Howard Brown Health, an LGBTQ-focused clinic in Chicago, reported eight new cases of mpox, formerly known as monkeypox, since April 17. By comparison, only one case, also diagnosed at Howard Brown, was reported to the Chicago Public Health Department in the previous three months. 

Public health experts expressed measured concern that, similar to other cases of infections that are transmitted through sexual contact, mpox cases could rise during the summer, especially as gay and bi men travel to Pride festivals and other major LGBTQ events.

“Without renewed vaccination and prevention efforts, we are at risk for a resurgence of mpox,” said Dr. Demetre Daskalakis, the deputy coordinator of the White House national mpox response. “Vaccine is a really important tool, even if it’s not perfect.” 

Diagnosed cases have been “increasing slightly” in eight countries in the past three weeks, including France and several countries in east Asia, said Dr. Rosamund Lewis, the technical lead for mpox at the World Health Organization. About half of the recent French cases were in vaccinated people.

Otherwise, global case counts have continued to decline since the early-August peak. Since mid-April, Lewis said, only 21 of the 111 nations to report cases during the outbreak have reported any new diagnoses. However, many cases may go undetected, she said.

Chicago’s weekly mpox case rate is the highest since early November. According to Centers for Disease Control and Prevention data, which is updated biweekly, the U.S. has had a rolling seven-day average of no more than 17 diagnoses since mid-February. 

The Howard Brown clinic is analyzing the new cases to determine whether any are part of a linked cluster of infections.

Mpox vaccination rates are insufficient

Seven of the eight newly diagnosed patients were in gay or bisexual men, said Dr. Leanna Gordon, the medical director of preventive medicine at Howard Brown. One person was unvaccinated, one had received one dose of the two-dose Jynneos vaccine, and six were fully vaccinated. 

All eight cases were mild, none of them requiring treatment for pain. Even with a breakthrough infection, the vaccine is still likely to reduce the severity of an infection, Gordon said.  

Gordon emphasized the importance that people at substantial risk of mpox get both doses of the Jynneos vaccine to protect themselves and their sexual partners.

“One of our major concerns is that our population at risk are under-vaccinated,” Gordon said. “We haven’t had as much interest in the vaccine as we would like.”

According to the CDC, more than 1.2 million doses of the Jynneos vaccine have been administered across the country to 725,000 people, 90% of them male. The agency estimates that only 1 in 4 of those most at risk, including gay and bi men and transgender people, have been fully vaccinated. 

Two doses of the Jynneos vaccine reduced the risk of mpox disease by 69%, and that one dose was 37% effective, the CDC reported in December. Updated figures are expected next week.

Public health experts expressed concern that mpox could spread more widely across the country after Chicago hosts the International Mr. Leather festival, a popular destination for gay men, at the end of the month. 

Daskalakis said the CDC is working with Howard Brown and other local partners to leverage the event to promote mpox prevention and awareness.

Dr. Jeffrey Klausner, an infectious disease physician at the University of Southern California, said, “We need to maintain vigilance and promote and provide free vaccination.”

Symptoms of mpox

Mpox, which has an incubation period of about three to 17 days, typically manifests as a rash and lesions. Other potential symptoms include:

• Fever. 
• Chills. 
• Swollen lymph nodes. 
• Exhaustion. 
• Muscle aches. 
• Headache.
• Sore throat.

In France, national health authorities recently reported that of the 19 diagnoses this year through April 3 in the Centre-Val de Loire region, 16 were made after March 1 and all but one were in men who have sex with men. Ten of the cases were in people who were fully vaccinated: six who had two doses of the Jynneos vaccine and four who had received the smallpox vaccine in childhood and a recent Jynneos dose.

Could mpox return this summer?

Last summer’s mpox outbreak declined following a combination of vaccination, immunity after infection and behavior change among gay and bi men. 

Infectious disease experts believe existing immunity from previous infection and vaccination would be likely to prevent mpox from spreading at the same level as last year.

But questions remain about the duration of immunity and whether gay men, many of whom reported reducing their number of sexual partners or even practicing abstinence last summer, have returned to their baseline behavior patterns after mpox cases dramatically waned.

“Everybody’s efforts have contributed to controlling the outbreak, but it’s not gone,” Lewis said. She noted that there is evidence that mpox can spread both before symptoms begin and from people who never develop symptoms.

Dr. Marc C. Shamier, a resident in clinical microbiology at University Medical Center Rotterdam in the Netherlands, reported that in September, about 45% of gay and bi men visiting sexual health centers in Rotterdam and Amsterdam had detectable antibodies against orthopoxviruses, the family to which mpox belongs.

“However,” Shamier said in an email, “this level of immunity is not sufficient to fully stop viral circulation, so sporadic clusters of monkeypox are to be expected.”

Shamier continued: “Large-scale events such as annual Pride parties could increase the number of sexual contacts among the risk group, which may lead to a rise in viral circulation and infections.” 

With testing, isolation of cases and higher levels of mpox immunity among risk groups, Shamier expects the number of cases to be relatively low compared to last year. 

Daskalakis said the recent cases in Chicago amount to “a call to action as opposed to a call to panic.” 

“We have the tools to take care of this,” he said. “We just have to get it done.”