The mRNA vaccines are designed to express the spike protein in a membrane-bound form; very little is found floating free. See https://www.deplatformdisease.com/blog/spike-protein-circula....
Additionally, the mRNA vaccines express a modified spike protein that is stabilized in its "prefusion" state (as seen on the virus itself before it binds with a cell), and the resulting complex seems to have somewhat fewer effects even when it does interact with a cell.
But, why not to be worried:
- It's a normal process and most cells will be replaced.
- The effects would be tiny, especially compared to an infection with the real thing. If this was causing widespread issues, we would know by now.
- All studies done so far indicate that only a very small amount will enter systemic circulation.
It's a classic risk tradeoff, and the risks of taking the vaccine are much smaller than the risk of getting the real infection (billions of viruses replicating vs. one-time dose, real spike protein vs. stabilized spike protein).