In December 2020, the FDA authorized emergency use of two coronavirus vaccines in the U.S., one made by Pfizer and BioNTech, and the other by Moderna. Both involve two doses, and both are made using messenger RNA, or mRNA, technology. Traditionally, vaccines have been made from a weakened or inactivated germ that trains the immune system to fight off infection if it encounters the virus in the future. But mRNA vaccines do something different: They teach human body cells how to make a harmless piece of a protein — a 'spike protein' — that's also found on the surface of the coronavirus. After that protein piece emerges on the surface of a cell, the human immune system recognizes it and begins making antibodies for it — which offer protection if the person is exposed to the actual virus in the future. Additional vaccine options are likely to become available soon. Johnson & Johnson is seeking FDA authorization for its single-dose vaccine, which has the advantage of being stored in regular refrigerators. A two-dose vaccine from Oxford and AstraZeneca is approved in the U.K. and European Union but not yet in the U.S.