The mRNA in the Pfizer-BioNTech vaccine encodes the spike protein of the SARS-CoV-2 virus. When the lipid nanoparticles deliver the mRNA into human cells, the cells' machinery translates it into the spike protein. This protein is then recognized by the immune system, which mounts a response by producing antibodies and activating other immune cells. If the vaccinated individual is later exposed to the actual virus, their immune system can quickly respond and neutralize the virus.
