There's an important detail here -- an advantage of mRNA vaccines is that mRNA does not last all that long in cells [1]. Thus, even if the cells live a long time, they won't produce spike proteins for very long (instructions for the production will be degraded and thus not available).
The vaccine's longer-term effectiveness comes from the immune system's memory B cells response to the short-lived expression of spike proteins.
you mean memory T cell right? As I understand it B cells are created by T cells to an acute infection.(they also kill infected cells but this is a gross simplification and i'm not a epidemiologist.)
The vaccine's longer-term effectiveness comes from the immune system's memory B cells response to the short-lived expression of spike proteins.
[1] https://en.m.wikipedia.org/wiki/Messenger_RNA#Degradation