The Food and Drug Administration authorized Pfizer’s Covid-19 vaccine for emergency use on Friday, clearing the way for millions of highly vulnerable people to begin receiving the vaccine within days.

The authorization was signed by the agency’s chief scientist on Friday evening, according to three people with knowledge of the decision who spoke on condition of anonymity because they were not authorized to discuss it.

Pfizer’s Covid-19 vaccine passed a critical milestone on Thursday when a panel of experts formally recommended that the Food and Drug Administration authorize the vaccine. The agency is likely to do so within days, giving health care workers and nursing home residents first priority to begin receiving the first shots early next week.

The F.D.A.’s vaccine advisory panel, composed of independent scientific experts, infectious disease doctors and statisticians, voted 17 to 4, with one member abstaining, in favor of emergency authorization for people 16 and older. Although the F.D.A. does not have to follow the advice of its advisory panel, it usually does.