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Myocarditis after mRNA COVID-19 vaccination

About half of the world’s population has received the COVID-19 vaccine. With cities continuing to open up, authorities are doubling their efforts to convince vaccine holdouts. Though vaccination have been shown to reduce hospitalization and death, many are still worried about potential side-effects from the COVID-19 vaccine. One such side-effect often heard on the news is vaccine-induced myocarditis.

Myocarditis is a condition where the heart muscle becomes swollen, affecting the heart’s ability to pump blood. Israel is one of the first countries to vaccinate a majority of its population with the BNT162b2 mRNA vaccine (Pfizer). After initial reports of an increased incidence of myocarditis, the Israeli Ministry of Health launched a surveillance program (1) to determine if there is a relationship between the vaccine and the occurrence of myocarditis.

Health records from December 2020 to May 2021 were screened for myocarditis diagnosis. After this initial screening, a board-certified cardiologist and rheumatologist evaluated the records and using the Brighton Collaboration Myocarditis definition, classified the cases as definitive, probable, possible, insufficient data or alternative diagnosis. Timing of the myocarditis diagnosis was also evaluated if it was 21 days after the first vaccine dose or 30 days after the second dose. Finally, the observed myocarditis occurrence rate was compared to the expected occurrence rate calculated from reported cases between 2017-2019 and to data from non-vaccinated residents.

What are the results?

More than 9 million residents were evaluated during the study period and 142 received a myocarditis diagnosis after vaccine administration. 136 were evaluated as definite or probable myocarditis, 1 as possible myocarditis and the remaining 5 cases had insufficient data. In comparison, there were 101 cases of myocarditis reported among non-vaccinated patients during the same period. Only 19 cases showed myocarditis after the first dose while 117 cases were reported after the second dose, with most cases occurring within 5 days after vaccine administration. 129 out of the 136 (95%) definite or probable myocarditis had short, uneventful hospital stays. Although there was one fatal case of fulminant myocarditis. There was no significant difference between observed (25) and expected (17.55) myocarditis occurrence after the first vaccine dose. However, there was an increase in myocarditis cases, especially among younger male patients, after the second dose (126 observed vs 23.43 expected). Furthermore, the risk of myocarditis in vaccinated residents (around 1 per 26,000 males and 1 per 218,000 females ) was more than twice compared to the non-vaccinated population.

What do the results tell us?

There seems to be an increased incidence of myocarditis after receiving a mRNA COVID-19 vaccine. The reason for this is still unclear. Cardiac tissue samples obtained using endomyocardial biopsy in another study showed lymphocytes infiltration but did not reveal any specific cause for the myocarditis (2). It is thought that the immune system in certain individuals may detect the mRNA in the vaccine as an antigen and triggers a systemic reaction leading to myocarditis (3).
The results also shows that most of the affected patients responded well to treatment and recovered quickly. Given the known complications from COVID-19 infection, risk-benefit ratio still remains favorable for vaccination. According to the Center for Disease Control and Prevention (CDC), for every million doses of COVID-19 vaccine to males aged 24-29 years old, 15-18 myocarditis cases might occur but it can prevent 15,000 COVID-19 cases, 936 hospitalizations, 215 ICU admissions and 13 deaths (4).  

While it is reasonable to be cautious,  evidence from various sources shows that getting vaccinated is still the best option to protect yourself and the people around you from severe acute respiratory coronavirus 2 (SARS-CoV-2) and its variants.  


1.     Mevorach, D., Anis, E., Cedar, N., Bromberg, M., Haas, E. J., Nadir, E., Olsha-Castell, S., Arad, D., Hasin, T., Levi, N., Asleh, R., Amir, O., Meir, K., Cohen, D., Dichtiar, R., Novick, D., Hershkovitz, Y., Dagan, R., Leitersdorf, I., . . . Alroy-Preis, S. (2021). Myocarditis after BNT162b2 mRNA Vaccine against Covid-19 in Israel. New England Journal of Medicine. Published.

2.     Lim, Y., Kim, M. C., Kim, K. H., Jeong, I. S., Cho, Y. S., Choi, Y. D., & Lee, J. E. (2021). Case Report: Acute Fulminant Myocarditis and Cardiogenic Shock After Messenger RNA Coronavirus Disease 2019 Vaccination Requiring Extracorporeal Cardiopulmonary Resuscitation. Frontiers in Cardiovascular Medicine, 8.

3.     Bozkurt, B., Kamat, I., & Hotez, P. J. (2021). Myocarditis With COVID-19 mRNA Vaccines. Circulation, 144(6), 471–484.

4.     Wallace, M., & Oliver, S. (2021, June 3). COVID-19 mRNA vaccines in adolescents and young adults: Benefit-risk discussion. CDC: Centers for Disease Control and Prevention. Retrieved November 12, 2021, from

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