Biden urges those eligible for a Pfizer booster to get one soon.

President Biden said Friday that coronavirus booster shots for millions of Americans who received the Pfizer-BioNTech vaccine will begin immediately and urged those eligible for a third shot to get one quickly to fortify their protection to the dangerous Delta variant that swept through the country this summer.

Mr. Biden spoke just hours after the director of the Centers for Disease Control and Prevention officially endorsed a Pfizer booster for older adults, many people with underlying health conditions, and frontline workers — like teachers and nurses — whose jobs put them at higher risk of contracting the disease.

People in those categories can get a booster if they received their second dose of the vaccine six months ago, Mr. Biden said.

“My message today is this: If you’ve got the Pfizer vaccine, you got the Pfizer vaccine in January, February, March of this year, and you’re over 65 years of age, go get the booster,” he said. “Or, if you’re in a have a medical condition like diabetes, or you’re a frontline worker like a health care worker or a teacher, you can get a free booster.”

Early Friday morning, Dr. Rochelle Walensky, the C.D.C. director, overruled a recommendation by her agency’s panel of scientific advisers that had refused to endorse booster shots of the Pfizer-BioNTech Covid vaccine for frontline workers. Dr. Walensky’s highly unusual move aligned C.D.C. policy with the Food and Drug Administration’s endorsements over her own agency’s advisers.

The C.D.C.’s Advisory Committee on Immunization Practices on Thursday recommended the boosters for a wide range of Americans, including tens of millions of older adults, and younger people at high risk for the disease. But they excluded health care workers, teachers and others whose jobs put them at risk. That put their recommendations at odds with the F.D.A.’s authorization of booster shots for all adults with a high occupational risk.

Dr. Walensky’s decision bolstered Mr. Biden’s campaign to give a broad segment of Americans access to boosters. The White House had come under criticism for getting ahead of the regulatory process. The C.D.C. panel was not asked to judge whether people who received the Moderna and Johnson & Johnson vaccines should receive the additional doses, which have not been authorized by the F.D.A.

The advisers also wrestled with the practicalities of endorsing a booster shot for only Pfizer-BioNTech recipients, when close to half of vaccinated Americans have received Moderna or Johnson & Johnson vaccines.

Asked on Friday what he would say to people who disregard the new C.D.C. guidance and get a booster shot despite being ineligible, Mr. Biden urged patience. Many Americans have already scrambled to get boosters even before federal regulators signed off this week, typically by finding a cooperative pharmacist or by claiming to be unvaccinated.

“You’re going to see that in the near term, we’re probably going to open this up anyway,” he said, referring also to the Moderna and Johnson & Johnson vaccines. “We’re also looking to the time when we’re going to be able to expand the booster shots, basically across the board. So I would just say, it’d be better to wait your turn in line, wait your turn to get there.”

Moderna’s booster authorization may arrive in a few days to weeks. The company has applied to the F.D.A. for authorization of a booster shot carrying half the dosage given in the first two shots, which has complicated the agency’s deliberations.

Some global health experts have criticized the Biden administration for pushing booster shots when much of the world has yet to receive a first dose. But analysts noted that even if the United States distributes booster shots, there should still be considerable excess vaccine supply this year, and they urged the government to begin sending the extra doses abroad.

In his remarks, Mr. Biden complained again about the resistance to the vaccine.

“It’s caused by the fact that despite Americans having an unprecedented and successful vaccination program, despite the fact that for almost five months free vaccines have been available in 80,000 locations, we still have over 70 million Americans who fail to get a single shot,” he said. “And to make matters worse, there are elected officials actively working to undermine with false information the fight against Covid-19.”

“This is totally unacceptable,” he said.

Dan Levin contributed reporting.