Originally Published: 15 MAY 23 15:20 ET

By Jacqueline Howard, CNN

(CNN) -- The US Centers for Disease Control and Prevention is raising the alarm about the potential risk for new cases of mpox, previously known as monkeypox, to emerge this summer.

"Spring and summer season in 2023 could lead to a resurgence of mpox as people gather for festivals and other events," the CDC said in an official update to its Health Alert Network on Monday.

The CDC is investigating a cluster of mpox cases in the Chicago area with its local partners, the agency said in Monday's health advisory, and the CDC "continues to receive reports of cases that reflect ongoing community transmission in the United States and internationally."

Twelve confirmed cases and one probable case of mpox were reported to the Chicago Department of Public Health between April 17 and May 5, according to the CDC. All of the cases were among men between the ages of 24 and 46. All had symptoms, but none of the patients has been hospitalized. Nine of those 13 cases were among men who had received both doses of the two-dose Jynneos mpox vaccine. Four of the men had recently traveled to New York, New Orleans and Mexico.

The CDC noted in its alert that although there can sometimes be cases among vaccinated people, vaccination can reduce the severity of the infection, lowering the chances of hospitalization.

"The purpose of this Health Alert Network (HAN) Health Update is to inform clinicians and public health agencies about the potential for new clusters or outbreaks of mpox cases and to provide resources on clinical evaluation, treatment, vaccination, and testing," the agency said.

'We remain watchful'

Some cities and counties, such as San Francisco, are encouraging people to get vaccinated to prevent a rise in cases this summer.

"Cases of mpox in San Francisco remain low, however, we remain watchful, as several new cases have recently been reported in other parts of the country," San Francisco Health Officer Dr. Susan Philip said in a news release last week. "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."

Mpox is a viral disease that spreads through close contact, and while it can infect anyone, during last year's outbreak, infections were mostly among men who have sex with men. The infection causes distinct rashes that can initially look like pimples or blisters, and other symptoms can include fever, chills, swollen lymph nodes, aches and fatigue. In rare cases, mpox can turn deadly, especially for people with weakened immune systems.

The potential risk of a rise in new mpox cases looms because there are still questions around how long vaccine-induced or natural immunity against the virus lasts, said Dr. Jay Varma, professor of population health sciences and director of the Center for Pandemic Prevention and Response at Weill Cornell Medicine.

"There's a very real risk of there being a surge in mpox cases over the next few months. Most likely, it won't be anywhere near as big as it was last year, and that is because many people at risk were either infected or vaccinated," Varma said.

"But we also know that there's also a lot of uncertainty about how long protection lasts from the infections or vaccinations that you had in the past, as well as the uncertainty about even more people potentially being at risk, because of course, every year, new teenagers become adults who are sexually active, and therefore they are likely at risk but were never vaccinated in the past," he said. "The primary way in which mpox is being transmitted in these outbreaks is through sexual contact."

He added that, overall, the vaccine is safe, is effective and played an important role in reducing transmission in last year's mpox outbreak.

"Even though there is some uncertainty about whether or not it's 80% or 90% or 100% effective and how long it's effective, we do know that you're better off being vaccinated than being unvaccinated," said Varma, who also serves on the board of directors at SIGA Technologies, the developer of the smallpox antiviral drug Tpoxx, which has been used and is being studied in clinical trials as a treatment for mpox.

'A grave concern'

Until the latest cluster of cases in Chicago, the United States was "down to maybe about one case a day, and those are narrowed to maybe one to two jurisdictions in the country," said Lori Tremmel Freeman, chief executive officer of the National Association of County and City Health Officials.

However, Freeman said, it's estimated that only 1 in 4 people at the highest risk for mpox is fully vaccinated.

"So there's a real concern that the vaccination level of the people most at risk is not where we need it to be, and we now are experiencing a new outbreak of cases in the Midwest that includes previously vaccinated individuals, although the symptoms appear to be milder for those who had some protection from vaccine," Freeman said.

"There's much more to unpack here," she said, "but the vaccine remains the best chance of avoiding severe disease, including hospitalization and death, and people should plan to get vaccinated two weeks ahead of attending any major events where there is risk of exposure to mpox."

This week marks the first anniversary of the first US mpox case in last year's global outbreak. The World Health Organization declared last week that mpox is no longer a global health emergency.

"However, as with Covid-19, that does not mean that the work is over," WHO Director-General Tedros Adhanom Ghebreyesus said last week.

"Mpox continues to pose significant public health challenges that need a robust, proactive and sustainable response," Tedros said. "While we welcome the downward trend of mpox cases globally, the virus continues to affect communities in all regions, including in Africa, where transmission is still not well understood."

Overall, "mpox is still a grave concern" and needs to be "back on the radar" of communities and health care providers, David Harvey, executive director of the National Coalition of STD Directors, said in an email Monday.

"One year ago this week, we experienced an unprecedented outbreak of mpox. The public health sector -- and especially those of us who work on STIs -- quickly rallied to push for a strong response. Ultimately, more than a million doses of the vaccine were distributed. Still, many people did not get vaccinated -- especially as case numbers dropped," Harvey wrote.

"Now in Chicago, we are seeing the resurgence of cases we anticipated seeing, and we continue to be concerned about wider spread throughout the summer and festival season," he said. "The good news is that we know what works and we have the tools at hand in the event of another outbreak to curb cases once again. The vaccine is safe and effective, and it helps reduce cases and lessen the severity of symptoms."