“According to the World Health Organization and cardiology societies, myocarditis is defined as an inflammatory disorder of the heart muscle that is characterized by lymphocytic and monocytic infiltrates within the myocardium, myocyte degeneration, and nonischemic necrosis (the so-called Dallas criteria).

It follows that for a truly definitive diagnosis of myocarditis occurring after vaccination, endomyocardial biopsy would need to be performed and viral myocarditis (including from Covid-19 infection) would need to be ruled out to exclude a chance occurrence of myocarditis temporally associated with the vaccine.

Most of the reported cases that occurred after vaccination had an uneventful course. The take-home messages from the two studies may be that clinically suspected myocarditis is temporally associated with the BNT162b2 mRNA vaccine but is rare, is more common in young male patients, and (with a few exceptions) is self-limiting. As acknowledged by the authors, temporal association does not imply causation, and the risk of vaccinal myocarditis is very low. The results of these two studies are valuable for doctors, patients, and the public to reduce the fear of myocarditis as a reason for excluding young people from vaccination, especially since myocarditis has also been temporally associated with Covid-19. Meanwhile, active surveillance for myocarditis should continue, and endomyocardial biopsy could be performed in severe cases to affirm the diagnosis and possibly to guide therapy, such as the use of antiviral drugs or immunosuppressive and immunomodulatory agents.”

More on Receipt of mRNA Vaccine against Covid-19 and Myocarditis via NEJM.

On the topic:
Mevorach D, Anis E, Cedar N, et al. Myocarditis after BNT162b2 mRNA vaccine against Covid-19 in Israel. N Engl J Med 2021;385:2140-2149.
Witberg G, Barda N, Hoss S, et al. Myocarditis after Covid-19 vaccination in a large health care organization. N Engl J Med 2021;385:2132-2139.
Caforio ALP, Baritussio A, Basso C, Marcolongo R. Clinically suspected and biopsy-proven myocarditis temporally associated with SARS-CoV-2 infection. Annu Rev Med 2021 September 10 (Epub ahead of print).

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