: the transmembrane anchor region*
"...when a cell gets the effect of an mRNA nanoparticle or an adenovirus vector, it of course starts to express the Spike protein. But instead of that being assembled into more infectious viral particles, as would happen in a real coronavirus infection, this protein gets moved up to the surface of the cell, where it stays. That's where it's presented to the immune system, as an abnormal intruding protein on a cell surface. The Spike protein is not released to wander freely through the bloodstream by itself, because it has a transmembrane anchor region that (as the name implies) leaves it stuck."
* About the author: "Derek Lowe, an Arkansan by birth, got his BA from Hendrix College and his PhD in organic chemistry from Duke before spending time in Germany on a Humboldt Fellowship on his post-doc. He’s worked for several major pharmaceutical companies since 1989 on drug discovery projects against schizophrenia, Alzheimer’s, diabetes, osteoporosis and other diseases."