mRNAs. "messenger" RNA are intended to be disposable copies of the information tucked safely away in the cell nucleus. As such we all create and recycle untold billions of them daily. The half life of a mRNA will be lucky if it is a day after about 5 halving you are approaching 1% of the total remaining. So a decade for a particular mRNA in an active cell is effectively infinity and stands no chance of remaining.

A question you may be thinking is then how can it help and the answer is in the new trick it teaches your cells to copy in those first few days, your body will remember the trick for a while, months maybe, a year hopefully, and that one cool trick is to recognize the virus so your body can do what it does with uninvited guests.

But the mRNA delivered as the vaccine is long gone, recycled back into pulp and made into new sticky post-it notes over and over.

The "trick" is the _information_ in the vaccine not the post-it note it comes on.

It is exactly what your body will try to find on it's own more or less at random when you get sick, not stumbling on the correct information in time can be fatal in this case.

If your body does find the right information to recognize the invader and you win your body will remember what worked, maybe for a while maybe forever.

This is why successful invaders will change themselves to avoid being remembered and eventually those changes will have unintended consequences (e.g. pandemic).