Pfizer said it expects to have the vaccines rolling out of its Kalamazoo, Michigan, facility on Sunday morning.

The first shipments are expected to arrive at 145 sites on Monday, 425 sites on Tuesday and another 66 sites on Wednesday, Gen. Gus Perna of the Trump administration's Operation Warp Speed vaccine program said.

At those sites, the vaccines will be paired with ancillary kits to assist with the vaccinations. The kits contain needles, syringes, alcohol swabs, diluent, safety gear and vaccine reminder cards.

ABC News' Luis Martinez contributed to this report.

Common questions surrounding the newly authorized Pfizer vaccine include safety for teenagers and the risk of allergic reactions.

Though the final data doesn’t have conclusive data on 16 and 17 year olds because they weren’t enrolled in trials until more recently, the FDA is “very comfortable with the safety profile that was observed in 17 and 16 year olds," Peter Marks, director of the FDA’s Center for Biologics Evaluation and Research, told reporters.

Regarding allergies, Marks said 1.6% of the population has had a severe allergic reaction.

Marks said you should notify your doctor if you’ve ever had an allergic reaction to a vaccine and noted that sites that administer the vaccine will have medications on hand to respond to an allergic reaction.

ABC News’ Anne Flaherty contributed to this report.

FDA Commissioner Stephen Hahn on Saturday denied reports he was threatened with firing.

Sources familiar with the matter told ABC News that in a Friday phone call, White House chief of staff Mark Meadows suggested to Hahn that his job could be on the line if his agency didn’t authorize emergency use of the Pfizer-BioNTech vaccine by the end of the day.

“Representations in the press that I was threatened to be fired if we didn’t get it done by a certain date is inaccurate,” Hahn told reporters Saturday.

Hahn said he would “absolutely” take the Pfizer-BioNTech vaccine. He said it was authorized late Friday because science and data said it was ready, not because of “any other external pressure.”

ABC News’ Anne Flaherty contributed to this report.

A record number of people -- 3,309 -- died from COVID-19 Friday, according to new data from Johns Hopkins University.

Friday's record shattered the previous single-day record death toll of 3,124 which was set Wednesday.

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.