As far as what happens years later, nobody really knows - as there's no history.

As I understand it, the mRNA ends up in the cytoplasm, not the nucleus, so it's unlikely to become a "permanent fixture in the body" - and those cells are destroyed by T-cells, anyway.

BTW, having T-cells attack those cells with mRNA that are producing foreign proteins will give you immunity more closely matching that of someone who's been infected. It's not clear that injecting killed virus into the bloodstream involves T-cells at all.

Disclaimer: non health-provider here.