• The CDC urged those at highest risk of mpox to get vaccinated.
• The CDC's call for those at risk to get up-to-date on their vaccines comes after a cluster of at least 21 mpox cases was reported in the Chicago area this month.
• Many of the people who caught mpox in the Chicago cluster were fully vaccinated against the virus, raising questions about whether immunity from the shots might wane off over time.

The Centers for Disease Control and Prevention on Thursday urged gay men and other individuals at high risk from mpox to get fully vaccinated to prevent a summer resurgence of the virus.

The CDC's call for those at risk to get up to date on their vaccines comes after a cluster of at least 21 mpox cases was reported in the Chicago area this month.

Many of the people who caught mpox in the Chicago cluster were fully vaccinated against the virus, raising questions about whether immunity from the shots might wane over time.

Dr. Demetre Daskalakis, deputy head of the White house mpox task force, said no vaccine is perfect but people who have received two doses have a much lower risk of catching and spreading the disease.

Three new reports released by the CDC and the New England Journal of Medicine Thursday demonstrated that two shots of the Jynneos vaccine provide more protection than a single dose.

The CDC estimated in one study that a single dose of the vaccine is 75% effective at preventing mpox, while two doses were about 86% effective. The New York State Department of Health found similar results in a second study, with one dose 68% effective and two doses about 88% effective.

But the New England Journal of Medicine study found that one dose was just 36% effective at preventing mpox, while two doses were 66% effective.

Though the estimates of mpox vaccine effectiveness vary, Daskalakis said the message is clear: "One dose is good, two doses are better."

"Now is the time to get vaccinated," Daskalakis told reporters on a call Thursday. "If you didn't get your first dose — get it. If you didn't get your second dose – get that," he said.

Less than a quarter of the 1.7 million people at highest risk from mpox – primarily gay, bisexual and other men who have sex with men – have received two doses of the vaccine.

Dr. Christopher Braden, the CDC's mpox incident manager, said health authorities are conducting studies to determine whether immunity after vaccination might wane off over time.

Braden said waning immunity is only one possible explanation for why an unexpected number of people in the Chicago cluster caught mpox despite being fully vaccinated against the virus.

He said the CDC is also studying whether the virus may have evolved over time to overcome immunity. It's also possible that the vaccine the patients in the Chicago cluster received were compromised in some way or weren't administered properly, he said.

This is a developing story. Please check back for updates.