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One or two doses of the Jynneos vaccine offered strong protection against infection from monkeypox, or mpox, compared with no vaccination at all, new data from the CDC showed.

Among vaccine-eligible men in 43 U.S. jurisdictions, mpox infection rates were 7.4 (95% CI 6.0-9.1) times lower with one dose of Jynneos and 9.6 (95% CI 6.9-13.2) times lower with two doses compared with no vaccination, reported Amanda B. Payne, PhD, of the CDC Mpox Emergency Response Team, and colleagues.

And "a proportional hazards model that accounted for time-varying risk supported the finding of a larger risk reduction among persons who had received two vaccine doses than among those who had received only one dose," they noted in the ).

The study also "provides additional data suggesting Jynneos vaccine provides protection against mpox, irrespective of whether the vaccine is administered intradermally or subcutaneously," Payne and colleagues wrote.

In order to extend vaccine supply, single doses of Jynneos were split into five doses and administered intradermally, but evidence on the effectiveness of the strategy has been limited.

Of the people with illness onset after at least one dose of vaccine, 87.1% had received subcutaneous administration and 12.9% had intradermal administration. These proportions were largely expected based on the percentages of people who received these routes of administration (83% and 17%, respectively), Payne and team said.

CDC also confirmed that the Jynneos vaccine is safe in a second looking at 1,350 reports to the Vaccine Adverse Event Reporting System (VAERS) from May 22 to October 21, a period when roughly 1 million Jynneos doses were administered.

Serious adverse events were rare, and no serious adverse events were seen in those under 18 years of age. Common side effects -- including injection site redness, swelling, pain, and dizziness -- were consistent with pre-licensure studies, reported Jonathan Duffy, MD, of the CDC Mpox Emergency Response Team, and colleagues.

"With a safe, effective vaccine in our hands, this is our chance to turn the page and finish strong," said Jonathan Mermin, MD, of the CDC Mpox Emergency Response Team, in a press release.

According to the , 29,711 cases have been reported in the U.S., but the outbreak has largely slowed, with an of six new cases per day. A total of 1,131,293 have been administered in 57 jurisdictions.

For their study, Payne and colleagues included data on 9,544 reported mpox cases among men ages 18 to 49 from July 31 to October 1. Men considered eligible for vaccination included gay, bisexual, and other men who have sex with men who have HIV infection or who are eligible to receive HIV pre-exposure prophylaxis. Vaccinated individuals in the study had to have received their Jynneos dose at least 14 days prior to infection for inclusion.

The researchers noted that the assumption that people with an unknown vaccination status were unvaccinated could have led to an underestimation of vaccinated people who were infected. In addition, it was not known if vaccinated men got the shot as pre-exposure or postexposure prophylaxis; therefore, some may have been exposed prior to vaccination.

Furthermore, the data do not account for risk behaviors (such as number of sexual partners) in people who were infected, or underlying medical conditions, such as HIV-associated immune suppression, and there may have been some men who were infected that were not part of the vaccine-eligible population.

Finally, since people were only considered vaccinated as of 2 weeks before the end date of a surveillance week, there may have been an underestimation of mpox in the vaccinated.

However, Payne and team noted, the analysis included the equivalent of over 1 million person-weeks of follow-up, and "expands knowledge about mpox incidence by vaccination status by including more jurisdictions during a longer observation period."

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