Nearly 1.5 million children in the U.S. have tested positive for COVID-19 since the pandemic began, the American Academy of Pediatrics and the Children’s Hospital Association said in its weekly report.

During the week ending Dec. 3, there were 123,688 new cases of children with COVID-19, bringing the total number of U.S. children diagnosed to 1,460,905.

ABC News' Eric Strauss contributed to this report.

Scientists from the U.S. Food and Drug Administration on Tuesday appeared set to confirm that the Pfizer/BioNTech COVID-19 vaccine is safe and effective against the virus, according to documents released ahead of a crucial meeting on the vaccine this week.

According to a document prepared by FDA staff, government scientists plan to tell the advisory committee that the data is adequate to determine the vaccine is both safe and effective.

While that doesn't mean the FDA is prepared to authorize it just yet, the briefing materials suggest agency staff aren't warning of any last-minute issues.

"Safety data from approximately 38,000 participants [greater than or equal to] 16 years of age randomized 1:1 to vaccine or placebo with a median of 2 months of follow up after the second dose suggest a favorable safety profile, with no specific safety concerns identified that would preclude issuance of an EUA," the FDA wrote.

The report notes the most common reaction was a skin reaction at the injection site, followed by fatigue or headaches.

The FDA released data on the Pfizer/BioNTech vaccine trials and the agency's scientific analysis ahead of an advisory committee meeting on Thursday, when independent experts are set to discuss and vote on whether to recommend the vaccine be authorized for emergency use.

ABC News' Stephanie Ebbs and Anne Flaherty contributed to this report.

The chief science adviser to Operation Warp Speed, the U.S. government's initiative to expedite vaccine development, said he doesn't know what vaccine-related executive order President Donald Trump is expected to sign on Tuesday.

"Frankly, I don't know and, frankly, I'm staying out of this. I can't comment," Dr. Moncef Slaoui told ABC News chief anchor George Stephanopoulos in an interview Tuesday on "Good Morning America."

When pressed on the matter, Slaoui added: "Our work is, you know, rolling. We have plans. We feel that we can deliver the vaccines as needed, so I don't exactly [know] what this order is about."

Trump is expected to discuss the order at a COVID-19 vaccine summit to be held at the White House later Tuesday, multiple White House officials told reporters during a background briefing on Monday evening. While it's not entirely clear on how exactly the order would work, the move is designed to prevent the U.S. government from shipping any vaccine doses it has purchased to aid foreign countries until all the needs within the United States are met.

White House officials also denied reports that the Trump administration turned down an offer last summer to purchase an additional 400 million doses of a COVID-19 vaccine developed by U.S. pharmaceutical giant Pfizer and its German partner BioNTech, and that the companies now may not be able to provide more of their vaccine to the United States until June 2021 because they have committed those doses to other countries.

When asked about the matter, Slaoui explained Operation Warp Speed's strategy in securing vaccine doses.

"We selected six different vaccines to build a portfolio to manage the risk that some may work and some may not work, but also to ensure that as more than one would work that we would accumulate vaccine doses from this portfolio of vaccines," he said. "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. Once the vaccine's performance becomes known is the right time, given the strategy we've taken, to go and order more vaccine doses, which we may be doing. And frankly, the constructive thing to do if one of the suppliers has challenges producing enough vaccine doses is to roll our sleeves and help ensure that capacity can be increased."

Slaoui said he's confident the U.S. government will be getting vaccines to Americans who need them "as soon as possible" and that plans are "still on track."

"We will work with Pfizer to try and increase capacity and have those vaccines available," he said. "We have two more vaccines from J&J and AstraZeneca that will be completing their Phase 3 trials in January and most likely, I hope, be approved for use in February. We have tens of millions of doses from those vaccines, you know, participating to the volume of vaccines we need to immunize the U.S. population as we promised, all of it by the middle of the year 2021 and that's still on track."

Slaoui said Operation Warp Speed has a meeting with President-elect Joe Biden's transition team scheduled for Thursday.

"We look forward to, you know, sharing all the information and working together," he said. "Our objective has always been outside of politics and making sure we make available these vaccines for the U.S. people, and that's what we're doing."

ABC News' Ben Gittleson, John Parkinson and Eric Strauss contributed to this report.

As COVID-19 vaccinations were administered to high-risk groups of people across the United Kingdom on Tuesday, Prime Minister Boris Johnson warned that the nation "can't afford to relax now."

"It is important for people to understand that the virus is alas still rising in some parts of the country," Johnson said while visiting a newly-opened vaccination center in London.

When asked whether he would be receiving the shot, Johnson told reporters he is "not in the priority group" outlined by the government but that those who are eligible should get it.

"I would just say to all those who are scared, don't be," he said. "You see people take the vaccine this morning, in large numbers, people are going to be taking it in the next few days and weeks and there is nothing to be nervous about."

The United Kingdom -- made up of England, Scotland, Wales and Northern Ireland -- has the seventh-highest tally of COVID-19 infections in the world, with more than 1.7 million confirmed cases, including over 61,000 deaths, according to a real-time count kept by Johns Hopkins University. The European island nation launched a mass immunization program on Tuesday, after authorizing emergency use of a COVID-19 vaccine developed by U.S. pharmaceutical giant Pfizer and its German partner BioNTech.

The first vaccinations in the state of Washington could be distributed as early as Tuesday, Gov. Jay Inslee said.

The initial shipment of the Pfizer vaccine arrives in the state on Monday.

"This is just fantastic news ... we are getting this vaccine," the governor said, adding he will get the vaccine himself when it is appropriate for his age group.

"Health care workers and long-term care residents will receive the vaccine first," he said.