“In a cohort study of 23.1 million residents across 4 Nordic countries, risk of myocarditis after the first and second doses of SARS-CoV-2 mRNA vaccines was highest in young males aged 16 to 24 years after the second dose. For young males receiving 2 doses of the same vaccine, data were compatible with between 4 and 7 excess events in 28 days per 100 000 vaccines after second-dose BNT162b2, and between 9 and 28 per 100 000 vaccines after second-dose mRNA-1273.”
Translation–for the mRNA-1273 second dose vaccine the rate is roughly 1 in 5000 among young men. And this is just one of many acknowledged possible adverse events.