(NEW YORK) — President Joe Biden on Tuesday announced a plan to distribute 500 million free at-home rapid tests to Americans beginning in January, doubling down on an effort to slow the spread of a highly transmissible variant that has hit the U.S. distressingly close to the holidays.

But he denied it was a failure not to have more tests already available as Americans wanted to get checked days before family gatherings.

“I want to start by acknowledging how tired, worried and frustrated I know you are,” Biden began in his address to the nation from the White House. “For many of you, this will be the first or even the second Christmas, we look across the table to see an empty kitchen chair.”

“If you’re not fully vaccinated, you have good reason to be concerned,” Biden went on, talking about the omicron variant, now the dominant strain in the U.S. “Almost everyone who has died from COVID-19 in the past many months has been unvaccinated. Unvaccinated.”

“You’re putting other people at risk — your loved ones, your friends, neighbors, strangers,” he said to unvaccinated Americans. “You may think you’re putting only yourself at risk. But, it’s your choice. Your choice is not just about you, it affects other people. You’re putting other people at risk. Your loved ones, your friends, neighbors, strangers you run into, and your choice can be the difference between life or death. The longer the virus is around, the more likely variants form that may be deadlier than the ones that have come before,” he said.

Still, the president rejected the idea that the U.S. would go back to March 2020 or impose new lockdowns, and said public health officials have the means this winter to keep schools open safely, for example.

“If you are vaccinated and follow the precautions that we all know well, you should feel comfortable celebrating Christmas and the holidays as you planned it,” he said.

“The best thing to do is get fully vaccinated and get your booster shot — and no, this is not March of 2020. Two hundred million people are fully vaccinated. We’re prepared. We know more. We just have to stay focused,” Biden added.

In his ongoing push to get hesitant Americans vaccinated and boosted, Biden also mentioned that former President Donald Trump this week said he had gotten his booster shot, adding, “Maybe one of the few things he and I agree on.”

He also gave the Trump administration a nod on vaccine development.

“Let me be clear, thanks to the prior administration and our scientific community, America is one of the first countries to get the vaccine,” Biden said. “Thanks to my administration and the hard work of Americans, we led a rollout that made America among the world leaders in getting shots and arms.”

Biden’s new efforts come as the omicron variant became the most dominant COVID strain in the country Monday, accounting for nearly three-quarters of all cases, and just as travel kicks off at nearly pre-pandemic levels for the holiday season.

The Biden administration’s free at-home rapid tests will be delivered by mail to Americans who request them, Biden said, marking a slightly different approach from European countries that chose to send tests to all residents.

“Because Omicron spreads so easily, we’ll see some fully vaccinated people get COVID, potentially in large numbers. There will be positive cases in every office, even here in the White House,” he said, one day after the White House reported the president came in contact with a staffer who later tested positive.

“But these cases are highly unlikely to lead to serious illness. Vaccinated people who get COVID may get ill, but they’re protected from severe illness and death. That’s why you should still remain vigilant,” he said.

Pressed by reporters, Biden rejected suggestions it was a failure to not have the tests readily available in advance of the holidays.

“No, it’s not a failure, but the alarm bell went off. I don’t think anybody anticipated that this was going to be as rapidly spreading as it did,” he said.

Americans will have to request the tests through a website that will launch in January, a senior administration official said on Monday, and it’s not yet clear how many tests Americans will be able to request per household.

The move is a significant departure from the White House’s posture just two weeks ago, when White House press secretary Jen Psaki dismissed the idea of mailing tests to every American as costly and wasteful.

“Should we just send one to every American?” Psaki told a reporter on Dec. 6.

“Then what happens if every American has one test? How much does that cost, and then what happens after that?” Psaki said.

But the effort shows the consensus among White House officials on the need to improve the nation’s testing apparatus, which was caught unprepared by the perfect storm of high demand for pre-holiday testing and a surge of omicron cases.

As a result, Americans have faced long lines and empty shelves this week as they attempt to safely gather for the holidays in keeping with CDC guidance, which calls for families to use at-home rapid tests as an extra layer of prevention before gathering.

Fielding questions after his remarks, Biden was also asked what took so long to get tests out to Americans as the spread of omicron coincides with a greater need for testing ahead of the holidays.

“Come on, what took so long?” Biden quipped back at a reporter.

“Well, what took so long, is it didn’t take long at all. What happened was, the omicron virus spread even more rapidly than anybody thought,” Biden said.

Dr. Sam Scarpino, managing director of pathogen surveillance at the Rockefeller Foundation and a member of their Pandemic Prevention Institute, said the government could have seen this coming.

“Scientists have been warning about the potential for new variants to come along for a year now or more. And we’ve known about omicron since the day before Thanksgiving. It’s been weeks at this point,” he said.

There will be other actions to get ahead of omicron outlined in Biden’s speech on Tuesday, too, like new federal testing sites around the country — the first several of which will launch in New York City by Christmas — and mobilizing 1,000 military doctors and nurses to overburdened hospitals, the senior administration official said.

The military aid will be five-fold the current deployment and could be a big boost to hospitals that have been running on fumes for two years.

There are currently 175 troops spread over four states, and since August 2021, when a joint military operation across the Army, Navy and Air Force began, about 530 medical military personnel have been deployed to work alongside civilian health care providers.

Tuesday marked the second time Biden addresses the nation about the omicron variant in less than a month.

In November, Biden announced a winter COVID plan that included beefing up vaccination and booster clinics to encourage more Americans to get protected and increase testing by getting insurance companies to reimburse the cost of at-home tests.

The 500 million free at-home tests that will be announced on Tuesday will be in addition to getting at-home tests reimbursed, which will go into effect on Jan. 15.

Noticeably missing from the new government efforts will be any attempts to enact further restrictions or lockdowns — which some European countries have opted for as omicron has swept through their populations.

“I know you’re tired. I really mean this, and I know you’re frustrated. We all want this to be over. But we’re still in it,” Biden said in closing. “This is a critical moment. But we also have more tools than we’ve ever had before. We’re ready. We’ll get through this. As we head into the holidays, I want us to all keep the faith.”

ABC News’ Anne Flaherty and Justin Gomez contributed to this report.

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