The FDA chief was summoned to the White House again Wednesday, an official said.

Despite his repeated efforts to take credit for the record pace at which scientists have developed a vaccine for the novel coronavirus, President Donald Trump was uncharacteristically silent Wednesday after the United Kingdom became the first nation to authorize a COVID-19 vaccine.

Trump has for months zeroed in on vaccine production as a key to fighting the virus — and to his reelection — pressuring federal regulators in the United States to move faster and alleging — without evidence — that they and pharmaceutical companies have slow-rolled the process in order to hurt him politically.

The White House chief of staff, Mark Meadows, on Wednesday summoned the commissioner of the Food and Drug Administration, Stephen Hahn, for a second meeting at the West Wing in as many days, a senior administration official told ABC News.

Trump has for months publicly pressured the FDA to act more quickly on authorizing treatments for COVID-19 — even when they had not been proven to be effective. During the campaign, he openly expressed his desire for a vaccine to become widely available by Election Day, which did not happen.

Shortly after the election, in a now rare public appearance on Nov. 13, he again bragged about the vaccine effort, which he dubbed “Operation Warp Speed,” calling it “unequaled and unrivaled anywhere in the world, and leaders of other countries have called me to congratulate us on what we’ve been able to do,” and he has recently lamented that President-elect Joe Biden would receive credit.

U.K. Prime Minister Boris Johnson, a Trump ally who recently bucked the president by recognizing Biden’s victory in the presidential election, noted his country was the first “to pre-order supplies of this Pfizer vaccine, securing 40 million doses.”

“All the time we’ve been waiting and hoping for the day when the searchlights of science would pick out our invisible enemy and give us the power to stop that enemy from making us ill,” Johnson said Wednesday. “And now the scientists have done it.”

The U.K.’s announcement Wednesday that it had granted a temporary authorization for emergency use of a vaccine developed and produced by Pfizer and the German company BioNTech shone a light on the different regulatory regimes there and in the United State

“I would expect the FDA to reach a similar conclusion,” the chief scientific adviser to the Trump administration’s vaccine program, Moncef Slaoui, told ABC News chief anchor George Stephanopoulos in an interview on “Good Morning America” Wednesday.

“The U.K. agency is a very, very high-level type of agency, comparable to the FDA in terms of quality and depth of its assessment,” he added. “But of course, as you know, the operation is totally separated from the FDA as far as judgments are concerned, and, you know, things will follow their course. But I hope by the 10th or 11th of December, the Pfizer vaccine is approved.”

The development of COVID-19 vaccines has moved at record speed, and the FDA has shortened its typical regulatory process significantly.

Top Trump administration public health officials have insisted that political motivations are not playing any role along the way, and White House press secretary Kayleigh McEnany on Wednesday portrayed Meadows meeting with Hahn as not unusual.

“It is par for the course for the chief of staff to meet with him as we’re trying to save American lives with the vaccine in record time,” she told reporters at a news conference.

The White House otherwise declined to comment on either of Hahn’s White House meetings this week.

McEnany also said that she was “not aware” of the White House receiving any heads up that the U.K. was going to authorize the Pfizer/BioNTech vaccine.

“I’m not aware of any heads-up that we were given,” she said Wednesday in response to a question from ABC News’ Ines de la Cuetara.

ABC News’ Katherine Faulders contributed to this report.