Thanks Walter for your question.

I'm aware of one mouse study (https://academic.oup.com/cid/article/74/11/1933/6353927), where intravenous (into the vein) injection of mRNA vaccine caused myocarditis (and spike protein expression on heart muscle cells). This effect was not seen with intramuscular (into the muscles) injection.

So, this finding led to the hypothesis that traces of mRNA vaccine entering the veins by accident during intramuscular injection might induce spike protein expression on heart muscles, triggering myocarditis.

The faster metabolism of young males and the pro-inflammatory effects of testosterone probably aid this process.