[scotty79]

I think they could replicate mRNA into latge batch, then test it if it conains enough of correct mRNA.

If there is some slightly incorrect mRNA it will result in cells producing wrongly folded protein and if there's enough of it you'll get additional immunity to something slightly different.

Cells that produce the protein are to be killed by immune system anyways and natural viral mRNA is probably not perfectly replicated 100% of the time.