With Pride events scheduled worldwide over the coming weeks, U.S. officials are bracing for a return of mpox, the infectious disease formerly called monkeypox that struck tens of thousands of gay and bisexual men worldwide in 2022. A combination of behavioral changes and vaccination quelled that outbreak, but a majority of those at risk have not yet been immunized.

On Thursday, the Centers for Disease Control and Prevention warned of a deadlier version of mpox that is ravaging the Democratic Republic of Congo and urged people at risk to be vaccinated as soon as possible. No cases of that subtype have been identified outside Africa so far. But the escalating epidemic in Congo nevertheless poses a global threat, just as infections in Nigeria set off the 2022 outbreak, experts said.

“This is a very important example of how an infection anywhere is potentially an infection everywhere, and why we need to continue to improve disease surveillance globally,” said Anne Rimoin, an epidemiologist at the University of California, Los Angeles.

Dr. Rimoin has studied mpox in Congo for more than 20 years, and first warned of its potential for global spread in 2010.

The C.D.C. is focusing on encouraging Americans at highest risk to become vaccinated before the virus resurges. The agency’s outreach efforts include engaging with advocacy groups and social media influencers who have broad appeal among the L.G.B.T.Q. community. In December, the agency urged clinicians to remain alert for possible cases in travelers from Congo.

There are two main types of mpox: Clade I, the type that is dominant in Congo, and Clade II, a versi