mRNA vaccines operate along what is known as the "central dogma of molecular biology" which was proven and published by Watson Crick in 1957-58.

The central dogma is this: a small segment of DNA is transcribed to mRNA, which is then translated to a corresponding protein, and it occurs in an irreversible process. This is how your body constantly manufactures what it needs, when it needs it.

Instead of giving you a weakened virus with the its entire genome, scientists have isolated a very specific segment of it which is unique to the virus, so that when your cell translates it into a protein, it is recognized as foreign and creates an immune response.

This is a safer method than injecting you with an entire virus genome, because the entire genome can use your cells to re-create entire viruses, which can then replicate more. Instead, mRNA vaccines only use a small segment of mRNA which cannot be used to create full viruses, just recognizable parts which your body will know to attack.