The overall risk is known, around 1 in 200,000 (0.0005%) Notably higher in males aged 15-30 or so at around 3 in 200,000 (0.0015%)
In April, 2021, international news media first reported rare cases of young men in Israel who had developed myocarditis shortly after vaccination with the Pfizer-BioNTech mRNA vaccine against SARS-CoV-2.1 Since then, many observational studies from Asia,2 Europe,3–5 the Middle East,6,7 and North...
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We also know the immediate danger to people, typically people needed hospital treatment with Ibuprofen or a stronger NSAID.
We don't know exactly what some people are affected. More research needed.
We don't know what if any the long term impacts might be. There is some signs of long term structural changes but no significant (ie measurable) decrease in health as a result. More monitoring is needed.
We also know that about 5% of COVID infections result in Myocarditis. If you're avoiding the vaccine out of fear of Myocarditis you're wrong.