How would that ever happen with an mRNA vaccine? Is that even possible? The half life of the spike protein is so short, any potential side effects would be manifested quite quickly. (weeks, at most a few months)
"For purposes of mRNA-based therapies, modifications have been developed to keep the molecule in cells long enough to allow for the therapy to be successful. But even with this, the mRNA will not lead to protein production for more than 10-14 days."