Vaccines are designed to be injected not into a blood vessel, but into the deltoid muscle (arm) or buttock, this means the vaccine diffuses slowly into the cardiovascular system, however, even the deltoid muscle contains small blood vessels, hence the technique of aspiration was devised, it means to pull back slightly after the needle has been inserted, to look for blood, if blood is seen in the fluid pulled back the needle has hit a blood vessel, so it is necessary to choose a new site, this technique prevents the vaccination from occurring into a blood vessel, and may prevent the clots and other adverse reactions.
This fella explains it: https://www.youtube.com/watch?v=hkopHLQjtVQ
There is a theory that inadvertent injection into a blood vessel may cause the serious thrombosis clots that have been seen with e.g. the AZ oxford vaccine.
So I insist on aspiration and after a bit of argument they do it.
Gee, the fact that these vaccines have been finding their way into the blood system is about 18 months old news - care of about 1,000 prize winning scientists. Perhaps it can be avoided by using alternative early, proven safe, cheap universally available treatments - oh but they've been made illegal in most first world countries. Meanwhile back at the ranch...