What changes are made to mRNA in these vaccines to make them last longer? Also wouldn't our immune system attack it as foreign then? It seems to me as a laypers

What changes are made to mRNA in these vaccines to make them last longer? Also wouldn't our immune system attack it as foreign then? It seems to me as a layperson that this must have been a key novel and clever part to make these vaccines work. I'd like to read more but don't know where to start.