The U.S. drug regulator on Friday granted an emergency use authorization for Pfizer Inc.'s COVID-19 vaccine, paving the way for the country that has suffered the world's highest numbers of coronavirus cases and deaths to begin a mass vaccination program.

The move comes just days after Britain started administering doses of the vaccine, becoming the first Western nation to do so. Canada's health regulator also approved the vaccine on Wednesday.

U.S. pharmaceutical giant Pfizer and its German partner BioNTech SE applied to the U.S. Food and Drug Administration on Nov. 20, supporting its bid with late-stage clinical trial data showing a 95 percent efficacy rate against COVID-19, the respiratory disease caused by the novel coronavirus.

Photo shows Pfizer headquarters in New York in November 2015. (Kyodo) 

Use of the vaccine has been proposed for individuals aged 16 and older, with two doses administered 21 days apart.

The FDA issued the authorization a day after an advisory committee agreed by majority vote that the benefits of the Pfizer-BioNTech vaccine outweigh its risks for use in the proposed individuals.

While it was widely expected that the FDA would greenlight the vaccine soon after the committee's approval, President Donald Trump, who has been eager to take credit for the swift development of vaccines that usually take years, had stepped up pressure on the regulator to take immediate action.

Calling the FDA "a big, old, slow turtle," Trump tweeted a message to the commissioner Friday, "Get the dam [sic] vaccines out NOW, Dr. Hahn @SteveFDA. Stop playing games and start saving lives!!!"

Trump later announced the news of the FDA authorization through a video posted on Twitter, saying that the shipping of the vaccine had already started and that the first dose will be administered "in less than 24 hours."

Health care workers and residents of nursing homes are to be vaccinated first.

"I promised that we would produce a vaccine in record time before the end of the year. They said it couldn't be done, but with today's announcement, we have now achieved that goal," said Trump, who has been largely criticized for his handling of the pandemic.

But there is no sign that the health crisis will immediately be brought under control. The daily death toll surpassed 3,000 on Wednesday, crossing the threshold for the first time since the virus accelerated in March in the United States, according to a tally by Johns Hopkins University.

The figure exceeds the number of victims killed in the Sept. 11, 2001 terror attacks on the United States.

Concerns also linger among the public on whether the vaccine is safe enough.

According to Health and Human Services Secretary Alex Azar, the United States, which has a population of some 330 million, is expected to have enough doses to vaccinate 20 million people by the end of the year, 50 million by the end of January, and at least 100 million in all by the end of the first quarter.

"We remain confident that across our portfolio of multiple vaccines, we will have enough doses for any American who wants a vaccine by the end of the second quarter of 2021," he told a press conference Wednesday.

U.S. biotechnology firm Moderna Inc. has also filed with the FDA for vaccine use authorization and an advisory committee meeting is scheduled next Thursday.

The Japanese government has an agreement with Pfizer to receive a supply of 120 million vaccine doses, enough for 60 million people, or roughly half its population, in the first half of next year.

It also has an agreement with Moderna for enough vaccine doses to cover 25 million people starting in the first half of next year.

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