evidence: Both the Pfizer-BioNTech COVID-19 vaccine (Comirnaty®) and Moderna COVID-19 vaccine (Spikevax™) use messenger ribonucleic acid (mRNA) to stimulate an immune response that can protect against future infection. Memorial Sloan Kettering infectious disease specialist Tobias Hohl discusses the technology of mRNA vaccines. [...] Traditional vaccines put a weakened or inactivated germ into our bodies. Messenger RNA (mRNA) vaccines, like the Pfizer and Moderna COVID-19 vaccines, teach cells how to make a protein that triggers an immune response if someone gets infected. When the vaccine is injected into the upper arm, the mRNA enters cells near the site of the injection and tells the cells to start making the same protein that is found in the COVID-19 virus. The immune system recognizes this protein and begins producing antibodies that can fight the virus if the vaccinated person is later infected. [...] The idea is that the first shot primes the immune system, helping it recognize the virus, and the second shot strengthens the immune response. Pfizer’s second shot is given three weeks after the first one; Moderna’s is spaced four weeks later. It is very important to have both doses of the same vaccine and follow the same procedures that made the clinical trials so successful.