…Compare this to what happens in vaccination. The injection is intramuscular, not into the bloodstream. That’s why a muscle like the deltoid is preferred, because it’s a good target of thicker muscle tissue without any easily hit veins or arteries at the site of injection. The big surface vein in that region is the cephalic vein, and it’s down along where the deltoid and pectoral muscles meet, not high up in the shoulder. In earlier animal model studies of mRNA vaccines, such administration was clearly preferred over a straight i.v. injection; the effects were much stronger. So the muscle cells around the injection are hit by the vaccine (whether mRNA-containing lipid nanoparticles or adenovirus vectors) while a good portion of the remaining dose is in the intercellular fluid and thus drains through the lymphatic system, not the bloodstream. That’s what you want, since the lymph nodes are a major site of immune response. The draining lymph nodes for the deltoid are going to be the deltoid/pectoral ones where those two muscles meet, and the larger axillary lymph nodes down in the armpit on that side.
Now we get to a key difference: 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. That’s how it sits in the virus itself, and it does the same in human cells. See the discussion in this paper on the development of the Moderna vaccine, and the same applies to all the mRNA and vector vaccines that produce the Spike. You certainly don’t have the real-infection situation of Spike-covered viruses washing along everywhere through the circulation. The Spike protein produced by vaccination is not released in a way that it gets to encounter the ACE2 proteins on the surface of other human cells at all: it’s sitting on the surface of muscle and lymphatic cells up in your shoulder, not wandering through your lungs causing trouble.
Some of the vaccine dose is going to make it into the bloodstream, of course.
