We have hundreds of years of experience with viral genetic vectors. There's a whole subfield of rheumatology dealing with autoimmune conditions triggered by viruses.

The mRNA vaccines are just a single protein not the whole virus and it never gets assembled so its using considerably less cellular machinery than a real virus would.

We've literally studied this shit for hundreds of years, and we're using less moving parts that would happen with a real infectious virus.

Cold fact: this is not unknown territory at all.