Traditional vaccines contain an attenuated or dead bacteria or virus to prompt the body to generate an immune response. But mRNA injections use messenger RNA - a type of RNA necessary for protein production. In the case of the SARS-CoV-2 mRNA vaccines, this mRNA is injected in order to use the human cells to produce spike proteins found on the virus's outer membrane. Using the injected mRNA, the body's own cells then produce the viral proteins. The immune system recognizes the protein as foreign and produces immune responses (including antibodies) against it. These immune responses then allow the immune system to respond quickly when subsequently exposed to the virus, at least theoretically.