WASHINGTON (AP) — President Donald Trump celebrated the expected approval of the first U.S. vaccine for the coronavirus Tuesday as the White House worked to instill confidence in the massive distribution effort that will largely be executed by President-elect Joe Biden

Trump said the expected approvals are coming before most people thought possible. “They say it’s somewhat of a miracle and I think that’s true," he declared.

Trump led Tuesday's White House event celebrating “Operation Warp Speed,” his administration's effort to produce and distribute safe and effective vaccines for COVID-19. The first vaccine, from drugmaker Pfizer, is expected to receive endorsement by a panel of Food and Drug Administration advisers as soon as this week, with delivery of 100 million doses — enough for 50 million Americans — expected in coming months.

“Every American who wants the vaccine will be able to get the vaccine and we think by spring we’re going to be in a position nobody would have believed possible just a few months ago,” Trump said.

Pfizer developed its vaccine outside of “Operation Warp Speed,” but is partnering with the federal government on manufacturing and distribution.

England began its first vaccinations earlier Tuesday, to great fanfare, as the world mounts its fight against the pandemic that has killed more than 285,000 Americans and some 1.5 million people worldwide.

Trump and his aides hope to tamp down skepticism among some Americans about the vaccines and help build the outgoing Republican president’s legacy.

However, Trump’s administration was also facing new scrutiny Tuesday after failing to lock in a chance to buy millions of additional doses of Pfizer’s vaccine, which has been shown to be highly effective against COVID-19. That decision could delay the delivery of a second batch of doses until Pfizer fulfills other international contracts.

Trump used Tuesday’s event to sign an executive order in which the secretary of Health and Human Services is directed to ensure that Americans have priority access to the vaccine.

A senior administration official said the order would restrict the federal government from delivering doses to other nations until there is excess supply to meet domestic demand, but it was not immediately clear what the practical impact would be.

Tuesday's “Operation Warp Speed” event featured Trump, Vice President Mike Pence and a host of government experts, state leaders and business executives, as the White House looked to explain that the vaccine is safe and lay out the administration’s plans to bring it to the American people. But officials from Biden’s transition team, which will oversee the bulk of the largest vaccination program in the nation’s history once he takes office Jan. 20, were not invited.

Biden, who was rolling out his senior health team on Tuesday, said last week that in meetings with Trump administration officials his aides have discovered that “there’s no detailed plan that we’ve seen” for how to get the vaccines out of containers, into syringes and then into people’s arms.

Trump administration officials insist that such plans have been developed, with the bulk of the work falling to states and local governments to ensure their most vulnerable populations are vaccinated first. In all, about 50,000 vaccination sites are enrolled in the government’s distribution system.

But career officials insisted it was still too early to declare victory.

““We don’t want to get out in front of ourselves,” said Army Gen. Gustave Perna, responsible for overseeing the logistical and distribution efforts. "As my father used to say, ‘You can only spike the football when you’re in the end zone.’ Well, what is the end zone described to us here? Shots in arms.”

Speaking in Wilmington, Delaware, Biden promised to distribute “100 million shots in the first 100 days" of his administration — roughly on pace with Trump's projections for vaccination.

Introducing his pandemic response team on Tuesday, Biden laid out his priorities for the start of his new government. He repeated his previous calls for all Americans to wear masks for 100 days to prevent the spread of the virus and said he’d mandate doing so in federal buildings and on public transportation. Biden also said he believed the virus could be brought under enough control to reopen “the majority of schools” within his first 100 days as president.

Those pledges came even as Biden struck a somber tone about the toll the coronavirus has already taken. He said that, after about nine months of living with the pandemic, the U.S. is “at risk of becoming numb to its toll on all of us” and “resigned to feel that there’s nothing we can do."

Trump, meanwhile, defended his decision to hold indoor holiday parties at the White House this December, though they have attracted hundreds of largely mask-less supporters contrary to his administration's warnings that the American public should avoid such settings.

“Well, they’re Christmas parties," he told reporters Tuesday.

Though Trump was taking credit for the pace of vaccine development, much of the groundwork was laid over the past decade, amid new research into messenger RNA, or mRNA, vaccines — of the sort developed by both Pfizer and Moderna.

“The speed is a reflection of years of work that went before,” Dr. Anthony Fauci, the top U.S. infectious disease expert, told The Associated Press this month. “That’s what the public has to understand.”

Fauci, who will serve as a chief scientific adviser to Biden's administration, appeared virtually at the president-elect's event, but did not attend the White House summit. The White House did include early clips of Fauci predicting a longer development time for the vaccines in a round-up of skeptics of Trump's timetable.

The Trump administration insists that between the Pfizer vaccine, the vaccine from Moderna and others in the pipeline, the U.S. will be able to accommodate any American who wants to be vaccinated by the end of the second quarter of 2021.

The Food and Drug Administration’s panel of outside vaccine experts is to meet Thursday to conduct a final review of the Pfizer vaccine, and it will meet later this month on the Moderna version.

FDA decisions on the two vaccines are expected within days of each meeting. Both have been determined to be 95% effective against the virus that causes COVID-19. Plans call for distributing and then administering about 40 million doses of the two companies’ vaccines by the end of the year — with the first doses shipping within hours of FDA clearance.

The decision not to secure additional Pfizer purchases last summer was first reported by The New York Times. Health and Human Services Secretary Alex Azar told NBC the administration is "continuing to work across manufacturers to expand the availability of releasable, of FDA-approved vaccine as quickly as possible. … We do still have that option for an additional 500 million doses.”

Dr. Moncef Slaoui, who is leading the government’s vaccine effort, noted the Trump administration had been looking at a number of different vaccines during the summer. He told ABC’s “Good Morning America” on Tuesday that “no one reasonably would buy more from any one of those vaccines because we didn’t know which one would work and which one would be better than the other.”

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AP writers Jonathan Lemire in Wilmington, Delaware, Lauran Neergaard and Kevin Freking in Washington and Linda Johnson in Trenton, New Jersey, contributed to this report.