Myocarditis is a generally mild inflammation of heart tissue which can cause scarring but is usually resolved within days. The increased incidence of myocarditis was seen primarily in males, who had been vaccinated with mRNA vaccines designed to elicit immune responses specifically to the SARS-CoV-2 virus. Heart Inflammation Increased Post COVID-19 Vaccine? One Lakh Males Aged 12 to 17 Experienced Myocarditis Within 21 Days After Receiving Second Dose, Says CDC.