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. With rare exceptions, the F.D.A. follows the advice of its advisory panels.