I recently saw a discussion about the issue of vaccine entering the blood stream. It was suggested that occasionally the needle might enter a blood vessel when the shot was being given. For protection against that possibility it was suggested the plunger should be withdrawn a little to see if any blood comes back into the syringe. If so then the shot should be stopped and reapplied in another spot for safety.
I saw that awhile back too ; I think it was a Dr. Campbell presenting, when I saw it.
I have seen other commentary that it doesn’t matter, simply because muscle is very highly perfused, and injecting into the muscle is bound to introduce a lot of microlipid containing the MRNA, directly into the bloodstream anyway.
...of course, it might be that the initial immediate amount into the blood matters too.