The Centers for Disease Control and Prevention late Thursday endorsed booster shots for millions of older and high-risk Americans, opening a major new phase in the U.S vaccination drive against COVID-19.

CDC Director Dr. Rochelle Walensky signed off on a series of recommendations from a panel of advisers hours after the advisers said boosters for the Pfizer-BioNTech COVID-19 vaccine should be offered to people 65 and older, nursing home residents and those ages 50 to 64 who have risky underlying health problems. The extra dose would be given once they are at least six months past their last Pfizer shot.

But Walensky decided to make one recommendation that the panel had rejected.

The panel on Thursday voted against saying that people can get a booster if they are 18 to 64 years of age, are healthcare workers or have another job that puts them at increased risk of being exposed to the virus.

But Walensky disagreed and put that recommendation back in, noting that such a move aligns with an FDA booster authorization decision earlier this week.

People who have gotten two shots and decide not to get a third will still be considered fully vaccinated, the CDC said.