Heart muscle inflammation or "myocarditis" has appeared in a small group of people, mostly men under 30, who recently received a COVID-19 mRNA vaccine.

The United States Centers for Disease Control and Prevention is monitoring the cases in young adults and said there is a "likely association" between the vaccines and reported cases of heart inflammation, but there is not a deep enough understanding of this issue to understand why vaccines may contribute to the condition. The group calls these events "important but uncommon" as they are highly rare.

Heart muscle inflammation, otherwise known myocarditis, is a condition caused most commonly by viral infections. It can also be caused by bacterial or fungal infections, autoimmune diseases, severe reactions to medicines, and exposure to certain environmental toxins. Common symptoms of myocarditis include chest pain, shortness of breath and heart palpitations. It most frequently occurs in young men, and those with pre-existing medical conditions. Myocarditis is a rare condition and is most commonly treated with medicines over a week to ten days. Very rarely, myocarditis can lead to hospitalization. 

In a recent press conference, the US Centers for Disease Control said that according to early vaccine safety data, myocarditis does appear to show up more frequently than normal in some groups fully vaccinated against COVID-19. The CDC highlighted that these numbers were in line with the clinical trial data based on which these vaccines were authorized for emergency use by the Food and Drug Administration.

It is important to note that myocarditis has also been linked to people who have been infected with COVID-19, in addition to other issues like blood clots, strokes, heart attacks, and more.

At this stage, the CDC continues to advise vaccination for young adults, with precautions to be taken for those who have pre-existing medical conditions, and those that report any symptoms of myocarditis after their first dose. Based on the data available at this time, the clinical prognosis for the small number of individuals who suffered from myocarditis post-vaccination were seen to be encouraging. 

The risk of hospitalization from COVID-19 is also an order of magnitude higher, even in younger age groups, than the risk of suffering from myocarditis post-vaccination. Based on the current evidence, the CDC recommends continuing with vaccinations for younger age groups including children. 

Some recently published studies have highlighted the slightly increased rate of heart muscle inflammation in children and young adults who have received the Pfizer-BioNTech and Moderna vaccines. These studies, published since the Food and Drug Administration approved the emergency use of these vaccines in adolescents in early May.