President Biden may have gotten ahead of the government’s scientists in announcing prematurely that virtually all Americans would begin getting coronavirus booster shots this fall, but he made a show of getting his own. The president spoke briefly before he received a Pfizer-BioNTech booster on Monday afternoon.

“Let me be clear,” Mr. Biden said before he got the shot. “Boosters are important. But the most important thing we need to do is get more people vaccinated. The vast majority of Americans are doing the right thing.”

His third shot came only days after federal regulators moved to allow millions of Americans to get Pfizer booster shots if individuals received a second dose of that vaccine at least six months ago and met new eligibility rules. Frontline workers, older people and younger adults with medical conditions or jobs that place them at higher risk got the green light following weeks of intense debate within regulatory agencies that left much of the American public confused about the specifics of the booster plan.

Mr. Biden, eligible for a booster at age 78, has been vaccinated in public before when he got his first Pfizer dose last December, a contrast to his predecessor, Donald J. Trump, who received an early vaccine at the White House but did not talk about it at the time. But Mr. Biden has pursued the opposite strategy.

The White House press secretary Jen Psaki said on Monday that Mr. Biden had gotten his booster on camera “to make clear it’s safe, it’s effective, it’s something you should do if you’re in one of these categories.”

Senator Mitch McConnell of Kentucky, the minority leader, announced on the Senate floor that he had also received a booster shot — “an easy decision,” he added, particularly as a survivor of polio.

On Monday, Mr. Biden added to the air of nonchalance around the booster by answering reporters’ questions — about critiques of vaccine policy, infrastructure negotiations and other topics — while getting injected.

World Health Organization officials have called for a global moratorium on booster shot programs until the end of the year, describing them as an unequal and ineffective use of the limited pool of available vaccines. Asked about the criticism, Mr. Biden reiterated that the United States had provided more vaccine doses to the global effort than all other countries combined, and would continue to do more.

Mr. Biden said that about 23 percent of adult Americans had not received a single dose of the vaccine, and they were causing “an awful lot of damage for the rest of the country.”

Asked what vaccination percentage would get things back to normal, Mr. Biden said he was not a scientist, but that so many people “can’t go unvaccinated and us not continue to have a problem.”

Mr. Biden said that he was moving forward with vaccination requirements where he could impose them, and that he planned to travel to Chicago on Wednesday to talk about individual businesses implementing their own vaccination mandates.

A Reuters/Ipsos national survey conducted Aug. 27-30 found that 76 percent of Americans who have received at least one shot of a vaccine want a booster. Only 6 percent do not, the poll found.

Mr. Biden asked on Friday for people who were not yet eligible to be patient. He said that his administration was “looking to the time when we’re going to be able to expand the booster shots, basically across the board,” and that boosters for the Moderna and Johnson & Johnson vaccines were likely in the offing.

“So I would just say, it’d be better to wait your turn in line, wait your turn to get there,” Mr. Biden said.

Zachary Montague and Emily Cochrane contributed reporting.