Interesting post, aspiration prior to intramuscular injections used to be done as the preferred injection site was the dorsogluteal muscle (bum muscle) which has the sciatic nerve and superior gluteal artery nearby. These days the deltoid muscle is used instead and does not have any major blood vessels running through the “triangle site”. https://www.cdc.gov/vaccines/hcp/acip-recs/general-recs/administration.html
Aspiration if done may sometimes show blood if the needle has punctured a small blood vessel/ capillary, however the risk of all the vaccine leaking entirely into a small vessel instead of staying in the muscle is low.
Intramuscular sites are preferred over intravenous sites for vaccines as they act as a depot and can slowly train the immune system. If the vaccine is injected directly into the vein, it gets degraded too fast and cannot stimulate a proper immune response. https://www.abc.net.au/news/science/2021-01-08/covid-19-vaccine-injection-upper-arm-muscle-deltoid-immune/13031918
At any rate, the antibodies formed from the vaccine will eventually migrate into the blood circulation and form part of the defence against CoVid-19 which attacks various organs.
