Acute myocarditis extremely rare after COVID mRNA vaccination in adults

Severe acute respiratory syndrome coronavirus-2 (SARS-CoV-2) is the causal agent of the coronavirus disease 2019 (COVID-19) pandemic and was first reported in Wuhan, China, in late 2019. SARS-CoV-2 is an RNA virus that belongs to the genus Betacoronavirus of the Coronaviridae family and has a high mortality rate. To date, it has claimed more than 4.8 million lives worldwide.

Scientists have worked at a record speed to develop various therapeutics and vaccines to rapidly reduce the mortality rate and contain the COVID-19 pandemic. Several SARS-COV-2 vaccines have received emergency use authorization (EUA) from various global regulatory bodies, and vaccination programs are being conducted in many countries.

Recently, many reports are surfacing regarding the development of acute myocarditis following COVID-19 Messenger RNA (mRNA) vaccine administration in adults.

Now, a new study published in the journal JAMA Internal Medicine evaluated the incidence of acute myocarditis and clinical outcomes among adults who received the mRNA vaccine in an integrated health care system in the United States.

About the Study

In this study, candidates, who were members of Kaiser Permanente Southern California (KPSC) and 18 years of age or above, were considered. All the candidates included in this study received at least one dose of the BNT162b2 (Pfizer) or mRNA-1273 (Moderna) mRNA vaccine between December 14, 2020, and July 20, 2021.

Based on the clinicians' reports to the KPSC Regional Immunization Practice Committee, researchers identified individuals who experienced post-vaccination myocarditis. These candidates were also determined based on the hospitalization reports, where they were diagnosed with myocarditis within 10 days of vaccination. All the candidates considered in this study were re-evaluated by at least 2 cardiologists. The authors of this study calculated the incidence ratio of myocarditis post-vaccination and compared the results with the unexposed group within the study period.

Main Findings

Scientists identified 2,392,924 KPSC members who received at least one dose of SARS-CoV-2 mRNA vaccines. In the study, 50% of the members received the mRNA-1273 (Moderna) vaccine, while the other half received the BNT162b2 (Pfizer) vaccine. Additionally, in this group, researchers observed that 54.0% were women, 31.2% White, 6.7% Black, 37.8% Hispanic, and 14.3% were Asian. They calculated the median age to be 49 years. Also, the study cohort consisted of 35.7% of individuals who were younger than 40 years. 93.5% of the candidates were completely vaccinated, i.e., had received two doses of an mRNA COVID-19 vaccine.

1,577,741 individuals belonged to the unexposed group, whose median age was found to be 39 years. In this group, 53.7% of individuals were younger than 40 years. This group included 49.1% of women and in terms of ethnicity, 29.7% White, 8.8% Black, 39.2% Hispanic, and 6.6% Asian individuals.

Researchers of this study reported that in the vaccinated group, 15 cases of confirmed myocarditis were present. Among these 15 individuals, two received the first dose of the vaccine, and the rest of the thirteen individuals were fully vaccinated.

Thereby, scientists inferred the incidence of 0.8 cases of myocarditis per 1 million first doses and 5.8 cases per 1 million second doses over a 10-day observation period. Interestingly, all the individuals who experienced myocarditis were men with a median age of 25 years. In the unexposed group, 75 cases of myocarditis were found during the study period. Among these, 52% were men with a median age of 52 years. The incidence rate ratio for myocarditis was calculated to be 0.38 for the first dose and 2.7 for the second dose.

The authors emphasized that all the patients who experienced myocarditis post-vaccination did not have a history of cardiac disease. Among the 15 individuals who suffered myocarditis following vaccination, eight received BNT162b2, and seven received mRNA-1273. Scientists further observed that among these 15 individuals, fourteen complained of chest pain within 1 to 5 days post-vaccination. In addition, all the myocarditis patients responded to treatment and did not require intensive care unit admission or readmission after discharge.


The main strength of this study is the demographic diversity of the study cohort. This study has some limitations, including its observational design, lack of myocardial biopsy for definitive diagnosis, non-uniform testing across all cases, limited follow-up periods, possible under-diagnosis of subclinical cases, etc. Owing to the observational nature of the study, the relationship between COVID-19 mRNA vaccination and post-vaccination myocarditis could not be established.

The authors also indicated that the prevalence of acute myocarditis among individuals who received a single dose or two doses of the COVID-19 mRNA vaccine is a rare occurrence. However, the current study indicated that young men are more susceptible to myocarditis post-vaccination, which requires further investigation for better clarity.

Journal reference:
Dr. Priyom Bose

Written by

Dr. Priyom Bose

Priyom holds a Ph.D. in Plant Biology and Biotechnology from the University of Madras, India. She is an active researcher and an experienced science writer. Priyom has also co-authored several original research articles that have been published in reputed peer-reviewed journals. She is also an avid reader and an amateur photographer.


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