evidence: The Pfizer-BioNTech and Moderna vaccines contain messenger RNA (mRNA), which is a type of genetic material. After a person gets the vaccine, the mRNA enters cells in the body and tells them to make copies of the COVID-19 virus’s "spike" protein (the protein that normally helps the virus infect human cells). This doesn’t cause disease, but it does help teach your immune system to recognize and attack the virus if the body is exposed to it in the future. [...] For people who do not have a weakened immune system, the CDC recommends the following, based on a person’s age and if they've received a COVID vaccine before: [...] The COVID-19 vaccines might not be as effective in people with weakened immune systems. This includes, for example, people who: