One year after a global mpox outbreak led to the first reported case on these shores, health officials are warning of the potential for a redux.

In an alert issued by the Centers for Disease Control and Prevention (CDC), the agency said that while cases of mpox (formerly monkeypox) have fallen since peaking last August, the risk for new clusters or outbreaks hasn’t gone away. 

“Spring and summer season in 2023 could lead to a resurgence of mpox as people gather for festivals and other events,” the CDC said.

The agency said it’s already investigating a cluster of mpox cases among fully vaccinated men in Chicago, raising questions about how long vaccine protection lasts. The cases, reported to the city’s health department from April 17 to May 5, included 12 confirmed and one probable case of mpox

Nine of the 13 were among men who were fully vaccinated with the two-dose Jynneos mpox vaccine. Four of the men had traveled recently to New York, New Orleans and Mexico.

The CDC said in its advisory that the vaccine can reduce transmission of the mpox virus and reduce disease severity, as well as lower chances of being hospitalized. But cases among the vaccinated are not unexpected.

Some cities and counties, like Los Angeles, took the mpox news as a cue to encourage people to get vaccinated ahead of the warmer months.

“While there have been only three confirmed new mpox cases reported since March 1 in Los Angeles County, the recently reported cluster of at least 14 mpox cases in the Chicago area highlights the high potential for mpox resurgence locally,” the Los Angeles County Health Department noted in a press release.

Ditto for San Francisco, whose health officer Dr. Susan Philip reminded the public that, “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 virus that typically spreads through close contact and anyone can be infected. During the global outbreak that began in May 2022, those primarily affected were gay, bisexual and other men who have sex with men, according to the World Health Organization.

The infection manifests with rashes that resemble pimples or blisters as well as fever, chills and swollen lymph nodes, among other symptoms. Although rare, death due to mpox is possible, especially among those with weakened immune systems.

The CDC’s notice came a week after the World Health Organization declared 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 cautioned during a press conference this month.

“Mpox continues to pose significant public health challenges that need a robust, proactive and sustainable response,” Tedros said during the presser. “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.”