WASHINGTON (AP) — President Joe Biden joked about his age Monday as he rolled up his sleeve for a COVID-19 booster shot, encouraging Americans to get vaccinated against the virus that has killed 688,000 in the U.S. and for those authorized to get their booster doses for more enduring protection.

Days after the Centers for Disease Control and Prevention and the Food and Drug Administration recommended a third dose of the Pfizer vaccine for Americans 65 and older and approved them for others with preexisting medical conditions and high-risk work environments, Biden said vaccination was essential to ending the pandemic.

"Let me be clear: Boosters are important, but the most important thing we need to do is get more people vaccinated," Biden said before getting the booster. He added that he did not have side effects after his first or second shots, and hoped for the same experience with his third.

Biden, 78, got his first shot on Dec. 21 and his second dose three weeks later, on Jan. 11, along with his wife, Jill Biden. Biden said the first lady, who is 70, would also receive the booster dose, but she was teaching Monday at Northern Virginia Community College, where she is a professor of English.

"Now, I know it doesn't look like it, but I am over 65 — I wish I — way over," Biden joked. "And that's why I'm getting my booster shot today."

Biden emerged as a champion of booster doses in the summer, as the U.S. experienced a sharp rise in coronavirus cases from the more transmissible delta variant. While the vast majority of cases continue to occur among unvaccinated people, regulators pointed to evidence from Israel and early studies in the U.S. showing that protection against so-called breakthrough cases was vastly improved by a third dose of the Pfizer shot.

But the aggressive American push for boosters, before many poorer nations have been able to provide even a modicum of protection for their most vulnerable populations, has drawn the ire of the World Health Organization and some aid groups, which have called on the U.S. to pause third shots to free up supply for the global vaccination effort.

Biden said last week that the U.S. was purchasing another 500 million doses of the Pfizer vaccine — for a total of 1 billion over the coming year — to donate to less well-off nations.

"So, we have plenty, plenty of opportunities to make sure we get everyone in the world — to play our part — the largest part in the world of getting everyone vaccinated," Biden told reporters.

Biden continued to take questions from reporters about his vaccination experience, as well as ongoing congressional negotiations about government funding and his efforts to pass both an infrastructure bill and an expansion of the social safety net through the House this week, as a military nurse injected the dose into his arm.

On Capitol Hill, Senate Republican Leader Mitch McConnell, 79, a polio survivor, encouraged Americans to get vaccinated and revealed he had also received a booster dose Monday.

"Like I've been saying for months, these safe and effective vaccines are the way to defend ourselves and our families from this terrible virus," he said.

Vice President Kamala Harris, 56, received the Moderna vaccine, for which federal regulators have not yet authorized boosters — but they are expected to in the coming weeks. Regulators are also expecting data about the safety and efficacy of a booster for the single-dose Johnson & Johnson shot soon.

At least 2.66 million Americans have received booster doses of the Pfizer vaccine since mid-August, according to the CDC. About 100 million Americans have been fully vaccinated against COVID-19 through the Pfizer shot. U.S. regulators recommend getting the boosters at least six months after the second shot of the initial two-dose series.

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Associated Press writers Josh Boak and Lisa Mascaro contributed.