He also signed an executive order aimed at ensuring the U.S. government obtained enough COVID-19 vaccine doses to inoculate Americans who chose to get vaccinated before sharing any of the supply with other countries or international entities.
Trump's remarks at the gathering, where scores of masked, top U.S. officials gathered for hours inside an auditorium on the White House complex, focused on hyperbolic praise for his administration's role in speeding up the vaccine production process.
A reporter pointed out to the president that "some of these scientific officials here in this room have encouraged Americans not to travel during this holiday season, not to go to large gatherings" but that "you have been holding holiday parties with hundreds of people, many not wearing masks."
Trump said he has seen "a lot of people wearing masks" at the parties and, referring to the size of the guest lists, added that "we've reduced the number very substantially." But in numerous videos of White House parties the past week, most attendees can be seen not wearing masks; the gatherings appear crowded, with guests packed together -- largely ignoring any attempt at social distancing.
The parties directly contradict the Centers for Disease Control and Prevention's advice to Americans that small, outdoor holiday gatherings -- or virtual get-togethers -- are preferable.
Asked if he had a message for Americans celebrating Christmas as the virus surges, Trump noted the CDC had guidelines but didn't elaborate -- before immediately pivoting to praising vaccines.
"Well, CDC puts out their guidelines, and they're important guidelines," he said. "But I think this: I think that the vaccine was our goal. That was No. 1, because that was the way -- that was the way it ends."
As the pandemic surges across the country, Trump downplayed ongoing spikes by attributing a rise in cases to increased testing. In reality, experts say, the sky-high rates of cases, hospitalizations and deaths are not explained just by more testing -- but rather increased spread of the virus through the nation.
The summit began with a highly political video that took comments out of context from Biden; the federal government's top infectious disease expert, Dr. Anthony Fauci; and former federal vaccine official Dr. Rick Bright, a whistleblower who was fired by the Trump administration before the Biden added him to his transition team. The video made it seem like those officials, and others, expressed pessimism about how quickly a vaccine could be produced -- in contrast with clips of Trump and Health and Human Services Secretary Alex Azar touting the administration's vaccine program, dubbed "Operation Warp Speed."
While the development of COVID-19 vaccines has taken place at a record pace, the United States had not exceeded expectations Trump and Azar themselves laid out earlier this year.
In the spring, the administration had spoken of obtaining 300 million doses by the end of the year, earlier this fall, Azar predicted there would be 100 million doses available by the end of the year.
In reality, the United States will obtain 40 million at most, and that's only if two leading vaccine candidates -- produced by the pharmaceutical company Pfizer and its German partner BioNTech, and a second by Moderna -- receive authorization by the Food and Drug Administration.
Executive order aimed at prioritizing Americans -- after Pfizer offer declined
Slaoui said in an interview on ABC"s "Good Morning America" that he was "staying out of this."
"Frankly, I don't know," Slaoui said, when ABC News Chief Anchor George Stephanopoulos asked him for details. "And frankly, I'm staying out of this. I can't comment. I literally don't know."
The order said that the U.S. government should not share any of vaccine doses it had obtained until it had enough for "all Americans who choose to be vaccinated." Until then, Trump ordered, the federal government should not send any doses abroad.
"Once we've ensured the ability to meet the needs of the American people, it would be, then, in the interests of the United States to facilitate international access to COVID vaccines," a senior Trump administration official told reporters Monday.
The White House on Tuesday did not respond to questions about whether the order would attempt to block Pfizer, which is based in New York, from fulfilling its contracts with other countries, although it did not appear designed to do so.
It was unclear whether that would even be legally possible. Pfizer is a multinational company with several manufacturing plants producing the coronavirus vaccine in the U.S. and Europe, including one in Belgium that handles the vaccine's final stages.
The order and summit came a day after reports emerged that the United States had over the summer declined to purchase additional doses of Pfizer's vaccine.
The administration had committed to buying 100 million doses -- enough to cover 50 million people, since the vaccine consists of a two-dose regimen -- but, according to a senior administration official, it had passed on the chance to lock in hundreds of millions more.
The White House denied the reports. Press secretary Kayleigh McEnany told Fox News on Tuesday that "it's just simply not true that they offered extra doses, according to the people who are involved in the negotiations."
But the administration's July announcement about its contract with Pfizer had noted that the drug company had left open the option for the U.S. "to acquire an additional 500 million doses."
Asked about the reports, Slaoui on "Good Morning America" defended the United States' approach. Under "Operation Warp Speed," the federal government committed to covering the cost of an initial tranche of doses from half a dozen vaccine candidates; for Pfizer, it said it would buy 100 million of its vaccine if the FDA authorized or approved it.
"We selected six different vaccines to build a portfolio to manage the risk that some may work and some may not work but also ensure that as more than one would work, that we would accumulate vaccine doses from this portfolio of vaccines," Slaoui said. "Now, in the summer, if somebody came to us and said, 'Let's buy more of this vaccine or that vaccine,' no one reasonably would buy more from any one of those vaccines, because we didn't know which one would work and which one may be better than the other."
White House summit aimed at self-congratulation
A senior Trump administration official told reporters Monday that Tuesday's White House gathering was intended to educate the public about the vaccine development process and "instill confidence" in it.
"We're going to have a vaccine that has efficacy that we can trust, safety that we can trust, and quality that we can trust," Dr. Peter Marks, the director of the FDA's Center for Biologics Evaluation and Research, said at the event. "And this is really important that we're only going to get there if we do this all together, by taking vaccines together. This is a time where we have to rebuild our trust in vaccines."
And there was another goal to the "summit," the official said: self-congratulation.
"It's really to congratulate the members of Operation Warp Speed, the members of the administration, the members of the scientific community, the members of the public sector, the U.S. military all coming together in, really, the most spectacular mobilization since World War II to get a vaccine to market much, much faster than has ever been done before," the official said.
Two leading developers of COVID-19 vaccines, the pharmaceutical companies Pfizer and Moderna, did not attend the "summit," and an official with Biden's transition team said Monday no one representing the incoming president had been invited.
Fauci, the federal government's top infectious disease expert, had said he could not attend due to a scheduling conflict.
Trump has lamented the fact that Biden might get credit for the vaccines, the distribution of which will primarily take place after Biden takes office next month.
He has remained uncharacteristically silent on the U.K.'s progress, after it last week authorized the vaccine produced by Pfizer and BioNTech, and as it started its mass vaccination campaign Tuesday.
U.K. Prime Minister Boris Johnson, a Trump ally who recently bucked the president by recognizing Biden's victory in the presidential election, on Tuesday visited a London hospital and described the experience of meeting a patient who had received a dose of the vaccine as "exciting" and "moving."
Vice President Mike Pence closed out Tuesday's summit predicting that the first vaccine candidate will be "likely approved this week."
"We are literally, we believe in all likelihood, just a few days away from the approval and distribution of the first coronavirus vaccine in America. That is warp speed," Pence declared.
But Pence also acknowledged the realities of the worsening pandemic -- as both positive cases and deaths per day in the United States reach record levels -- calling it "a time of renewed vigilance" while urging Americans to "be patient."
"The American people should know that we all still have work to do. Every single one of us has a role to play with cases rising and hospitalizations rising in virtually every jurisdiction, around America," Pence said. "It's time once again to redouble our efforts to put the health of our families, our neighbors and our communities first."
ABC News' Guy Davies, Anne Flaherty and Katherine Faulders contributed to this report.