The mpox vaccination campaign after an outbreak swept the United States last year was highly effective at preventing infection, new federal data showed.
One dose of the JYNNEOS vaccine was 75% effective and two doses of the vaccine were 86% effective, a new study published Thursday by the Centers for Disease Control and Prevention found.
Among patients who are immunocompromised, one dose of the vaccine was 51% effective and two doses were 70.2% effective.
CDC officials said the results showed the importance of at-risk populations getting vaccinated or getting a second dose among those who've only been partially vaccinated ahead of summer.
As of March 31, nearly 31,000 mpox cases and 42 deaths have been reported since the outbreak struck last year, according to the report.
During the height of the outbreak, cases were, in some instances, doubling every week. Although anyone -- regardless of sexual orientation -- is at risk if they have direct contact with an infected patient, the outbreak was primarily concentrated in men who have sex with men, a group that includes people who identify as gay, bisexual, transgender and nonbinary.
In response, health officials began advising anyone with known exposure to mpox or who might be at risk of exposure to get vaccinated with the JYNNEOS vaccine, which was approved by the Food and Drug Administration in 2019.
To increase the number of JYNNEOS doses available, the FDA authorized a proven strategy in August to inject the vaccine intradermally, just below the first layer of skin, rather than subcutaneously, or under all the layers of skin.
This allows one vial of vaccine to be given out as five separate doses rather than a single dose.
Since summer 2022, more than 1.2 million doses have been administered. However, there has been limited data on the effectiveness of the vaccine, so the CDC conducted a study comparing vaccine effectiveness among sexually active men who have sex with men or transgender people between ages 18 and 49.
Between August 2022 and March 2023, a total of 309 patients who were vaccinated were compared with 608 patients who were not.
Not only was the vaccine shown to be effective, but also there was little difference in vaccine effectiveness when it came to the method by which the shot was administered, whether subcutaneously or intradermally.
"Vaccination is an important tool for preventing mpox, and this report demonstrates that the JYNNEOS vaccine is effective at reducing risk for mpox," the report said. "However, additional pre-cautions to reduce exposure should be considered, particularly among immunocompromised persons" including limiting the number of sexual partners and one-time sexual encounters."
The study comes as the CDC and the Chicago Department of Public Health reported a recent outbreak of mpox.
Twenty-one cases in Chicago have been diagnosed, with most among gay, bisexual or other men who have sex with men, the CDC said during a press briefing with reporters Thursday morning.
Some cases did occur among previously vaccinated individuals, but there could be explanations including decreased immunity over time or infected patients being immunocompromised, the officials said.
This demonstrates "the ongoing risk for new cases and outbreaks and the need for continued vigilance and prevention efforts," Dr. Christopher Braden, the CDC mpox response incident manager, said during the briefing.