Myocarditis is a disease that causes inflammation of the heart muscle, the myocardium. The inflammation, often in response to a viral infection, “enlarges and weakens the heart, creates scar tissue and forces it to work harder to circulate blood and oxygen throughout the body.”  Pericarditis is a condition that affects the pericardium, a thin membrane that surrounds the heart.
According to the Myocarditis Foundation, the condition is rare. It can affect anyone of any age but is seen twice as often in males than females. Globally, 3.1 million cases of myocarditis were diagnosed in 2017. The condition can lead to heart failure.
Viral infections are a leading cause of myocarditis. “Inflammation occurs during the course of infection, putting stress on the heart that remains even after the infection is resolved,” according to the Foundation. Additional risks for myocarditis include exposure to environmental toxins, infection (bacterial, fungal, parasitic) and certain medications.
Symptoms of myocarditis include shortness of breath, fatigue, heart palpitations, chest pain, swelling of extremities. Or, symptoms may be non-specific. Diagnosis may require ECG and ultrasound.
Medical literature points to cardiac myosin as an autoantigen in autoimmune myocarditis and viral-induced myocarditis. Research has shown an increase in antibodies against myosin after infection with coxsackie virus in animal models. 
A study of the risks of myocarditis following the mRNA vaccination and infection with SARS-CoV-2 was recently published in Circulation, a journal of the American Heart Association. In it, researchers concluded that the risk of the condition is greater after infection. However, “… the risk of myocarditis after vaccination is higher in younger men, particularly after a second dose of the mRNA-1273 vaccine.” 
While the reported risk of myocarditis after COVID-19 vaccination is very low, this is based on the number of patients who went to the ER to evaluate new symptoms. The incidence of low-grade myocardial inflammation is unknown. This can occur without symptoms, and the potential increase in long-term risk of congestive heart failure that may result from this is also unknown.
Most cases of myocarditis are self-resolving. For some, recovery may require months with medical treatment and can recur.