https://blogs.sciencemag.org/pipeline/archives/2021/05/04/sp...
“Consider what happens when you’re infected by the actual coronavirus. We know now that the huge majority of such infections are spread by inhalation of virus-laden droplets from other infected people, so the route of administration is via the nose and/or lungs, and the cells lining your airway are thus the first ones to get infected. The viral infection process leads at the end to lysis of the the host cell and subsequent dumping of a load of new viral particles – and these get dumped into the cellular neighborhood and into the bloodstream. They then have a clear shot at the endothelial cells lining the airway vasculature, which are the very focus of these two new papers.
Compare this, though, 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.”