Although there have been some reports of myocarditis, or inflammation of the heart muscles, they are very rare compared to the hundreds of millions of vaccines given. In almost all cases, it is mild and resolves quickly.

In fact, myocarditis is a much more common side effect of COVID-19 than it is of the vaccine.

SOURCES:
https://www.cdc.gov/coronavirus/2019-ncov/vaccines/safety/adverse-events.html (CDC)
https://www.hematology.org/covid-19/vaccine-induced-immune-thrombotic-thrombocytopenia (American Society of Hematology)
https://www.cdc.gov/coronavirus/2019-ncov/vaccines/safety/myocarditis.html (CDC)

Updated 12/16/2021