Updated: Jul 2, 2021
Myocarditis is a risk factor in mRNA vaccines, one in about 230,000 but it is documented. My theory is that those affected had the vaccine injected mistakenly into a vein rather than soft tissue. It happens when those administering the vaccine do not aspirate before delivering the dose to check if they've hit a vein or not.
If a vaccine is delivered into soft-tissue you get a sore arm the next day due to the immune response (not trauma). If the vaccine goes into a vein and therefore the circulatory system it will find it's way to the heart and an immune response can commence there instead, hence myocarditis. I'm talking specifically about mRNA vaccines, however this is a risk-factor for any vaccine. I have not yet found any credible studies exploring this but it does seem to make sense. [EDIT] There have now been published studies which are under peer-review and it looks like failure to aspirate before delivering a vaccine not only explains myocarditis, but also cases of thrombosis. It's a fucking simple thing to do which Nadhim Zahawi is being criminally negligent on. Watch this video if you want it explained (not a huge fan of John Campbell but he's bang on with this video): https://youtu.be/e3lx6Scwfhg
Or just read the sources and make your own mind up.