Pfizer and Moderna require individuals to get two shots in order to be fully effective, with an additional booster shot suggested 6 months after your second shot. The Pfizer-BioNTech vaccine consists of two doses 21 days apart, and the Moderna vaccine has two doses 28 days apart. The Johnson & Johnson vaccine was approved for emergency use and requires only one dose to prevent Covid-19, although it is less effective than the Pfizer and Moderna vaccines.