WATCH: Biden reassures vaccinated Americans, issues dire warning to unvaccinated

WASHINGTON, DC -- President Joe Biden aimed to reassure Americans that if they are vaccinated, they can still proceed with their holiday plans without fear of becoming seriously ill.

"I know some Americans are wondering if you can safely celebrate the holidays with your family and friends. The answer is, yes you can if you and those you celebrate with are vaccinated, particularly if you've gotten your booster shot," Biden said in a White House speech Tuesday.

Conversely, he warned those who have not yet received shots of the potential for severe illness or death in the coming months.

"All these people who have not been vaccinated, you have an obligation to yourselves, to your family and quite frankly ... to your country. Get vaccinated now, it's free, it's convenient and it saves lives," he said.

Biden announced Tuesday the purchase of a half-billion at-home rapid Covid-19 tests, one of a series of new steps he unveiled in a speech that came as the country faces a potentially crippling wintertime surge of infections.

The 500 million new tests will be made available next month and will reach Americans through the mail, the official said. The administration is still working to determine how many tests each household may request. In a call on Tuesday, senior administration officials were unable to provide any new details about exactly when the website to request the tests will launch and how quickly tests will be shipped out.

"We're working through all the details. And we'll have those in the coming weeks," a senior administration official said during a briefing with reporters.

The official said the "first deliveries" of those tests will happen in January and that the website will launch "in January or around January as well." The official also said the White House is "working through all the details" when asked how Americans without internet access could secure tests.

The new test shipment scheme came during remarks meant to underscore the White House's plan to confront what is expected to be a large spike in cases driven by the now-dominant Omicron variant in the coming weeks. Already, cases are rising rapidly in areas around the country and hospitals are nearing capacity.

Among the President's new initiatives is a plan to prepare 1,000 military service members to deploy to overburdened hospitals across the country in January and February, administration officials said. Those service members will include doctors, nurses, medics and other military medical personnel.

An official said, "God willing" the full 1,000 wouldn't be needed, "but if we do, they are ready and they're mobilized."

"The President will tell the American people if they're vaccinated and follow the precautions we all know well, especially masking while traveling, they should feel comfortable celebrating Christmas and the holidays as they planned," a senior administration official said ahead of time, previewing a speech that Biden hopes will reassure anxious Americans ahead of the festive season.

By contrast, Biden warned the tens of millions of Americans who have so far declined to get shots that they run a high risk of becoming ill or hospitalized as the highly transmissible Omicron variant spreads rapidly across the country.

Biden also announced new vaccination sites and increased vaccine access, and will deploy hundreds of additional federal vaccinators who will help add "thousands of appointments each week," the official said.

Though demand for vaccines has dwindled in recent months, the surge caused by the Delta variant earlier this year did lead to increased demand for vaccinations in some of the hardest hit areas.

Biden also addressed FEMA efforts to expand hospital capacity and to pre-position medical supplies in places where stocks may run low in the coming weeks.

The speech, coming just days before Christmas, reflects Biden's renewed focus on the coronavirus pandemic as anxiety rises around the country at the potential for new restrictions and lockdowns.

Lines for testing centers have stretched for blocks in certain areas, including the northeast, leading the White House to develop plans to open new federal testing sites in New York City before Christmas.

Distributing free at-home tests to all Americans was written off by the White House as recently as this month. Press secretary Jen Psaki scoffed earlier this month when asked if tests should be free and given out, available everywhere.

"Should we just send one to every American?" she asked.

While the White House plan announced Tuesday does not ship test to every American, people would request the tests online instead. Those who utilize them will not have to report their results to public health agencies, officials said.

Pressed on that shift, a senior administration official insisted that the new plan "builds" on the administration's previous testing initiatives and pointed to increased supply.

"For the first time in the course of this pandemic because of all the actions we've taken -- because of FDA authorizing a lot of tests, because we created the market, because we've used the (Defense Production Act) consistently for the last 11 months -- we have the ability to make this purchase. We have the manufacturing capacity for a massive purchase like that starting in January," the official said. "If there's more we can do, we will do it."

The official also pointed out that "demand for tests ... has spiked now" amid the Omicron surge and the holiday season and said that the government's purchase of 500 million tests is an extra layer on top of existing commercial supply.

The administration had already announced a plan requiring health insurance providers to reimburse plan-holders for the cost of at-home tests; that initiative remains in place next year.

In his remarks, Biden acknowledged a likely rise in cases -- including among vaccinated people -- but emphasized the drastically different outcomes for those with shots and those unprotected by vaccines.

Biden and his team have been adamant that federal lockdown recommendations aren't in the cards this time. And in his remarks, Biden will focus on steps primarily to forestall a crippling of the US public health system than on efforts to fully stop the spread of the virus.