Straight from the CDC's website....
Myocarditis is inflammation of the heart muscle, and pericarditis is inflammation of the outer lining of the heart. In both cases, the body's immune system causes inflammation in response to an infection or some other trigger. 
I wonder what this other "trigger" might be.🤔 #pureblood
https://www.cdc.gov/coronavirus/2019-ncov/vaccines/safety/myocarditis.html