I just saw in an article, that while uncommon, mRNA vaccines can actually lead to circulating spike protein. From the article [1]:
Most notably, the plasma of patients with postvaccine myocarditis had “markedly elevated levels” of free spike antigen (33.9 ± 22.4 pg/mL), with the protein circulating in the body unbound by antibodies. “Although postvaccine myocarditis clinically occurs more commonly in males, [this] was seen equally in both affected females and males,” the investigators point out.
From the original study though, patients without myocarditis were found to have no such levels.
> A notable finding was that markedly elevated levels of full-length spike protein (33.9±22.4 pg/mL), unbound by antibodies, were detected in the plasma of individuals with postvaccine myocarditis, whereas no free spike was detected in asymptomatic vaccinated control subjects (unpaired t test; P<0.0001).
So what we have is a potential explanatory mechanism (namely, probably some biochemistry which means for whatever reason these people seem to release the protein rather then keep it cell bound - although correlation is not causation, the whole effect could work the other round).
Most notably, the plasma of patients with postvaccine myocarditis had “markedly elevated levels” of free spike antigen (33.9 ± 22.4 pg/mL), with the protein circulating in the body unbound by antibodies. “Although postvaccine myocarditis clinically occurs more commonly in males, [this] was seen equally in both affected females and males,” the investigators point out.
[1] https://www.tctmd.com/news/free-spike-protein-mrna-covid-19-...