Thank you for bringing that up. After a brief reading, it seems that IM injection for medication and vaccine may be different. For vaccine, aspiration before IM injection isn't necessary because no large blood vessels are present at the arm injection site. Still, this is just what I've read. I'm no doctor or nurse, so I'm not sure if things differ in actual practice.

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5333604/