I'm not sure of any direct scientific evidence supporting this, but I believe it has to do with the vaccine entering the blood stream. The serum is meant to be

I'm not sure of any direct scientific evidence supporting this, but I believe it has to do with the vaccine entering the blood stream. The serum is meant to be administered at an intramuscular site, but injections have a chance of hitting a blood vessel upon insertion. I am not a medical professional, but the advice I've read is that practitioners should "aspirate" the needle (pull back on the plunger) before delivering the vaccine, to see if they've hit a blood vessel. https://www.youtube.com/watch?v=KgVsd6qoyU4 https://www.youtube.com/watch?v=KgVsd6qoyU4 This theory somewhat matches the observations because young people are likely to have more, larger blood vessels in their arms compared to older demographics. This also explains why men experience myocarditis more than women.