Can anyone speak to the confidence we have in these claims? That is, are we virtually 100% sure that these mRNA have no mechanism to stick around for longer than intended?
GP's point is valid - that this is the first mRNA vaccine that would be widely used.
Let's say our assumption is wrong - these mRNA have a way to stick around forever. Is there an off switch?
With a bit of organic chemistry knowledge you can determine how the additional OH in RNA allows for water to attack there and cleave the RNA strand. This is very dependent on pH, but RNA is inherently less stable than DNA.
But that's not actually the most important mechnanism here, but it is one that is inherent in the chemistry of the molecule, there is no way around it.
Cells need to regulate protein production up and down. If mRNA were to live eternally, there would be no way to reduce protein production. So cells obviously have a way to remove mRNA. What actually happens is that mRNA is permanently degraded by RNAses, so any mRNA that isn't replenished will be gone after a while and protein production will stop. This is a fundamental part of regulation of protein biosynthesis, and you can find that in every textbook.