Why do different vaccines require a different amount of shots?

Different types of vaccines work in different ways to build protection. The Johnson & Johnsons] Jansen vaccine is the only available vaccine that has been found to be effective after one shot. Both the Pfizer-BioNTech and Moderna vaccines require two doses given several weeks apart. The first shot helps your body begin to build protection. The second shot a few weeks later gives your body the most protection the vaccine has to offer. Some individuals may benefit from an additional dose of these vaccines, especially if they have a condition that severely weakens their immune system. Please follow up with your healthcare provider for additional information on whether you need more than two shots. You are considered fully vaccinated two weeks after the last shot.