LONDON—The U.K. became the first Western nation to grant emergency-use authorization for a Covid-19 vaccine, clearing a shot developed by Pfizer Inc. of the U.S. and BioNTech SE of Germany to be distributed in limited numbers within days.

The two-shot vaccine is also being reviewed by the Food and Drug Administration in the U.S., where a similar authorization could come later this month and a rollout before the end of the year.

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The U.K. green light punctuates a monthslong sprint by the two drugmakers, which teamed up earlier this year and then pulled ahead of two other Western pharmaceutical giants, each with its own promising shot. Vaccines typically take years to bring to market.

It also marks a key milestone in efforts to translate a promising new vaccine technology into a widely available shot. It was developed, tested, authorized and is now poised to be distributed amid a pandemic that has sickened tens of millions of people and killed more than 1.4 million around the world.

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The U.K. has ordered 40 million doses, enough to vaccinate 20 million people. The government said in early November it expected to receive 10 million doses before year-end. People familiar with the matter said the U.K. might only get four million to five million does this year, but the number is in flux and will depend on production and other potential regulatory authorizations.

This story continues in The Wall Street Journal.