About 50 million people in the United States are expected to receive a coronavirus vaccine by January. But some believe that the number could effectively be doubled to 100 million without any additional production of shots, accelerating the return to normal.

Both of the Pfizer and Moderna vaccines, recently approved by the Food and Drug Administration, are supposed to be given in two doses, the first one a prime and the second a booster. The two doses of the Pfizer vaccine are to be given 21 days apart, with the Moderna at 28 days apart.