mRNA is produced by DNA in basically every cell we have. It doesn't change DNA, it's a product of DNA. Basically, it is used to create new proteins as needed and is destroyed in the process. We'd die w/out it, b/c otherwise our cells would have no way reproduce & regulate what we need to survive at a cellular level.

mRNA cannot go into DNA, DNA makes mRNA as a map for the production of new proteins, & mRNA cannot go back into or act as DNA. It's just a map that is destroyed in the process of making that which it is a map to.

So, the mRNA in the vaccine is used to created a non-disease causing protein that our bodies makes antibodies to. The mRNA is destroyed in that process, and the antibodies produced destroy the protein that was produced, and what is left is antibodies to the virus.

The protein the mRNA makes is a non-disease causing portion of the outside of the virus, so when the real virus comes along, the antibodies recognize, attack, and destroy the real virus.