The first rigorously tested coronavirus vaccine was given a green light for use Wednesday in Britain. Doses of the vaccine, made by the U.S. pharmaceutical giant Pfizer and a small German company, BioNTech, will be injected starting next week, the government said.

In leaping ahead of the United States to allow mass inoculations, Britain added to the pressure on U.S. drug regulators, who were summoned to the White House on Tuesday by President Donald Trump’s chief of staff to explain why they were not ready to do the same.

Why did Britain authorize a vaccine before the U.S.?