In this case, the mRNA just gets your body to produce a part of the spike protein that is on the virus. Once the spike protein is floating around in your body,

In this case, the mRNA just gets your body to produce a part of the spike protein that is on the virus. Once the spike protein is floating around in your body, your immune system recognizes the spike protein and starts an immune response. The spike proteins themselves cannot actually replicate and attack the body. They are just a small part of the virus structure. If you inject with virus, presumably the virus would begin to replicate and attack your body.