Not just myocarditis. There has been records of arthritis and even uveitis.
Vaccination (any . not just mRNA) elicits an immune response to produce protective antibodies . In certain individuals this escalates into an overwhelming auto-immune response resulting in a runaway cascade with the antibodies attacking own tissue. This can even happen without vaccines in a normal flu. Spontaneous myocarditis during influenza is well documented and somethings leads to heart failure and death.
https://www.straitstimes.com/life/t...apore-suffers-covid-19-myocarditis-and-stroke
In the above case the parents chose
not to vaccinate for fear of myocarditis but instead the child got it through the covid virus
In most instances the vaccine injury is self limiting and can be aborted with corticosteroids .
Even the fool of a teenage who tried to kill himself by going to the gym lifting heavy weights after vaccination didn't manage to die