White House chief of staff Mark Meadows hinted to Food and Drug Administration commissioner Stephen Hahn on a phone call Friday that his job security might be in jeopardy as he pushed the FDA chief to approve Pfizer's coronavirus vaccine by the end of the day, according to two sources familiar with the call.

Why it matters: It's one more example of the White House putting political pressure on the FDA to expedite its green light on a coronavirus vaccine.

On Twitter Friday, President Trump pressured the agency publicly as well. He called it "a big, old, slow turtle," and demanded: "Get the dam vaccines out NOW, Dr. Hahn @SteveFDA. Stop playing games and start saving lives!!!"

• In a statement, Hahn called the account “an untrue representation of the phone call with the Chief of Staff. The FDA was encouraged to continue working expeditiously on Pfizer-BioNTech’s EUA request. FDA is committed to issuing this authorization quickly, as we noted in our statement this morning.”
• A White House official replied, "We don't comment on private conversations, but the Chief regularly requests updates on progress toward a vaccine."
• The exchange was first reported by the Washington Post.

Where it stands: Pfizer's vaccine authorization was already on a fast track and the FDA's announcement was expected to come soon, perhaps over the weekend, following an advisory committee's recommendation on Thursday.

• Moving that timeline up by a day or two is unlikely to make a difference in how quickly people can actually receive the vaccine.

What's next: The administration has said that roughly 3 million doses of Pfizer's vaccine can be shipped immediately, once the FDA issues its authorization.

• Those doses will primarily go to health care workers. And because the Pfizer vaccine must be stored at sub-Arctic temperatures, it likely will be concentrated among research institutions that are equipped for ultra-cold storage.

Editor's note: This story has been updated with Trump's tweet and a response from Hahn.