CDC and other regulators working to assess risk and if there is a causal relationship between mRNA vaccines and myo/pericarditis seen predominately in a small number of males 16-24 years old after 2nd dose.

Medical Mythbusting Commentary for June 11, 2021

Source:
Heart inflammation in young men higher than expected after Pfizer, Moderna vaccines: U.S. CDC

Reference:
FDA COVID-19 Vaccine Safety Updates