All mRNA vaccines in use currently rely on modified nucleosides that cannot be replaced via our polymerases. Pseudouridine I think is the one.
Also we are relying on mRNA vaccines being manufactured correctly, but that link in the chain of reasoning also doesn't seem to be too reliable as many studies seem to have shown, here's one of the more recent ones: https://pubmed.ncbi.nlm.nih.gov/40913499/