Basically title. I tried to Google a little, but all results were about COVID Vax side effects, and apparently r/AskDocs has specific formatting rules for posts that I don’t feel my question necessarily requires, so just trying here unless someone has a better subreddit to try.

I recently had blood work done and had an IV, but I don’t recall having soreness or anything similar. Yesterday morning, I received my COVID vax (J&J), and was told my arm might be sore for a day or two. After I got home in the late afternoon/early evening, my arm had finally started to feel a little sore.

Why does one cause soreness at the site, but the other doesn’t? And is there any reason it took 8-ish hours for the soreness to start, or is that normal?