The U.S. Centers for Disease Control and Prevention is urging people at high risk of mpox to get two doses of Bavarian Nordic's Jynneos vaccine, based on new evidence from a U.S. study showing that the regimen is more effective at preventing infection than one shot.

The study, published on Thursday, offered some of the first evidence on the efficacy of the Jynneos vaccine, which was deployed last year during a global outbreak of mpox that affected more than 30,000 people in the United States.

The study of the vaccine's real-world use, conducted between Aug. 19, 2022 and March 31, 2023 among 917 people, showed it was 85.9% effective overall in preventing disease after two doses, compared with 75.2% after one dose.

For people in the study who were immunocompromised, the vaccine 70.2% effective after two doses and 51% for a single dose, but neither figure was statistically significant, according to the study published in the CDC's Morbidity and Mortality Weekly Report.

The outbreak, which started in May 2022, disproportionately affected gay, bisexual, and other men who have sex with men and transgender persons.

The recommendations follow a health warning to U.S. physicians on Monday about the risk of a resurgence in cases this spring and summer as people gather for gay pride festivals and other events.

"Without renewed prevention efforts, especially vaccination, we are definitely at risk of a resurgence, in fact, a substantial risk of resurgence of mpox," Christopher Braden, mpox response incident manager for the CDC, told reporters in a medial briefing.