Pfizer’s vaccine debuts in Britain as the world’s first fully tested vaccine.

Britain’s National Health Service delivered its first shots of the Pfizer-BioNTech Covid-19 vaccine on Tuesday, opening a mass vaccination campaign with little precedent in modern medicine and making Britons the first people in the world to receive a clinically authorized, fully tested vaccine.

At 6:31 a.m. Tuesday, Margaret Keenan, 90, a former jewelry shop assistant, rolled up the sleeve of her “Merry Christmas” T-shirt to receive the first shot, and her image quickly became an emblem of the remarkable race to produce a vaccine and the global effort to end a pandemic that has killed 1.5 million people worldwide.