evidence: Myocarditis is inflammation of the heart muscle while pericarditis is an inflammation of the membrane around the heart. They can occur as very rare adverse events after vaccination with mRNA vaccines. Cases typically occur within 10 days, with symptom onset often within 5 days of vaccination. Pericarditis symptoms may occur later, typically 2 to 3 weeks after vaccination. Myocarditis and pericarditis are often mild, and symptoms usually resolve after a short time with standard treatment and rest. Some cases are more serious and need to be treated in hospital, but very few cases require intensive care. Myocarditis has most often been reported after the second dose in 12 to 17 year old boys and men under 30. Several countries have observed higher reporting rates of myocarditis with those vaccinated with the Moderna COVID-19 vaccine than the Pfizer vaccine, but the reported difference in rates has differed between studies and countries may be influenced by a variety of factors. The benefit-risk of both of the vaccines remains positive. Pericarditis following a mRNA vaccine tends to occur at an older median age than myocarditis, but it is nonetheless more common in people under 50 years of age than in older people. Myocarditis and pericarditis can occur after a booster dose, but this is so far reported to be more rare than after the primary doses. There is no indication that these events are more serious than after earlier doses.