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 is a historic turning point in a pandemic that has taken more than 290,000 lives in the United States. With the decision, the United States becomes the sixth country — in addition to Britain, Bahrain, Canada, Saudi Arabia and Mexico — to clear the vaccine. Today, we ask the science and health reporter Donald G. McNeil Jr. what might happen next. Guest: Donald G. McNeil Jr., a science and health reporter for The New York Times. For an exclusive look at how the biggest stories on our show come together, subscribe to our newsletter. You can read the latest edition here. Background reading: Pfizer has a deal with the U.S. government to supply 100 million doses of the vaccine by next March. Under that agreement, the shots will be free to the public.The vaccin...