Okay, so let's reason this out. What would cause a coin -- or magnet -- to stick to an injection site for a short period of time, before not sticking?

Magnets stick to ferrous metals. Coins don't stick to anything that I can think of -- unless your skin is greasy, as Dave Morgan suggests! But let's stipulate that the skin surface is clean and dry.

What is the magnet sticking to?

Can you see how this is unfolding as an Internet hoax? Some "influencer" has probably posted a video and it's gone viral, shared by followers who find critical thinking too much a chore. I can see how those with naturally greasy skin (eww) might fall for this, as the coin might just stick. The coin would also have stuck the day before the first injection...but he never checked. In this regard, it is similar to the whole "egg-only-stands-on-its-end-during-the-spring-equinox" baloney.

If coins stick to your body, take a shower and get back to us. Actually, same goes for magnets.