WASHINGTON (AP) — The head of the agency responsible for approving any COVID-19 vaccine for the U.S. was summoned to the White House Tuesday as an increasingly frustrated President Donald Trump complained approval wasn't coming faster.

White House Chief of Staff Mark Meadows called in Stephen Hahn, the head of the Food and Drug Administration, for a meeting as the agency weighed whether to allow emergency use of the first vaccines that could help defeat the coronavirus.

Trump has been livid with the FDA for not moving faster on the vaccine, blaming it in part for his reelection loss. He has also leveled unfounded claims that drug companies deliberately delayed vaccine development to hinder his chances.

Tuesday's meeting came as the FDA weighed whether to authorize two experimental vaccines that have been raced through development. Many Americans already harbor concerns about the potential impact of political pressure on vaccine development, and public health officials have been trying to provide reassurance that the approval process has been free from influence.

Meanwhile, a separate scientific panel was tackling the pressing question of who should come first f or vaccinations when they become available.

The government’s Advisory Committee on Immunization Practices will make recommendations to the Centers for Disease Control and Prevention on shots to fight the disease, which has killed nearly 270,000 Americans.

The White House did not immediately respond to requests for comment about the purpose of the meeting meeting with the FDA's Hahn. But many aides to the president recognize that it would look bad for Trump to lose Hahn on the cusp of vaccine authorization. And some expressed concern Tuesday that the White House meeting was even happening, for fear that it could undercut efforts to frame the vaccine as a positive part of Trump’s legacy.

But the president has been fuming since he lost his reelection bid and believes that, had a pair of drug companies released data showing that their vaccines were effective before Election Day, he would have have won. He has complained repeatedly about not getting the credit he believes he deserves for the vaccine development.

Hahn said the FDA will thoroughly review each vaccine and not be rushed before making it available to the public.

“We will make sure that our scientists take the time they need to make an appropriate decision,” Hahn said, in a statement provided by the FDA. “It is our job to get this right and make the correct decision regarding vaccine safety and efficacy.”

An FDA spokesman said the agency must review “thousands of pages of technical information” provided by vaccine developers to ensure the shots were studied and manufactured properly.

A scientific researcher and former hospital executive, Hahn has clashed with Trump repeatedly amid the president's unsubstantiated suspicions that FDA scientists were working to undermine him.

To the contrary, in the first months of Hahn's tenure, former FDA staffers and outside experts criticized the FDA chief for decisions that many viewed as bowing to the White House and failing to protect the agency’s science-based decision-making process. More recently Hahn has resisted White House pressure for hasty vaccine approval.

In October, Hahn published strict FDA safety guidelines that effectively closed the door on Trump’s goal of delivering a vaccine by Election Day. And he has repeatedly testified to Congress that “science, not politics,” would determine when a vaccine was made available.

Trump's anger with the FDA and drug companies only escalated after the election, according to White House aides and a Republican close to White House who spoke on condition of anonymity to discuss private conversations. And Trump has been telling confidants that the vaccine is still being slow-walked, post-election, to hurt his chances of contesting the outcome.

For months, he has told friends that he believes Hahn slowed down or undersold the benefits of other therapeutics — including blood plasma — despite evidence to the contrary — and has “never been on the team.” He has also claimed both privately and publicly that drug companies are punishing him for his efforts to lower prescription drug prices, and has made unsupported allegations that Pfizer and others have formed an informal Deep State-like cabal with the FDA and the media to hurt him.

Amid his anger, he has been telling allies that he was assured the vaccine would be out by October and now believes it was either deliberately delayed or that he was given incorrect information to make him look bad and hurt his chances of winning.

Meanwhile, the administration has invited leading vaccine manufacturers, distributors and others to a “vaccine summit” next week. Not all invited companies have committed to attending for fear of being perceived as overtly political or being subjected to Trump's attacks.

Though Trump doesn’t give the FDA or Hahn credit, agency scientists were the ones who came up with the idea for “Operation Warp Speed,” the White House-backed effort through which millions of doses of coronavirus vaccines and treatments have been manufactured even as they are being evaluated.

That process — in which money is spent to produce vaccines that may not work — is aimed at cutting months and even years off normal production timelines.

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Miller reported from Wilmington, Delaware. Lemire reported from New York.