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Disclosures: The authors report no relevant financial disclosures.
October 04, 2021
2 min read

While rare, myocarditis after COVID-19 vaccination is more common in young men

Disclosures: The authors report no relevant financial disclosures.
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Acute myocarditis was rare in individuals who received at least one dose of the Pfizer-BioNTech or Moderna COVID-19 vaccine, according to findings published in JAMA Internal Medicine.

Among the few participants who had confirmed myocarditis after vaccination, researchers reported that all were men with a median age of 25 years.

Graphical depiction of data from study.
Simone A, et al. JAMA Intern. 2021;doi:10.1001/jamainternmed.2021.5511.

“Given the observational nature of this study, no causal relationship between COVID-19 mRNA vaccination and postvaccination has been established,” Mingsum Lee, MD, PhD, a cardiologist at Kaiser Permanente Los Angeles Medical Center and a clinician researcher with the Kaiser Permanente Southern California Department of Research & Evaluation, told Healio Primary Care. “At this time, the mechanisms for myocarditis with COVID-19 mRNA vaccines are not clear, and the reasons for male predominance are not known. Sex hormone differences could play a role, but certainly more research is needed to better understand the underlying mechanism.”

The researchers conducted a population-based cohort study of 2,392,924 adult Kaiser Permanente Southern California members who received at least one mRNA vaccine dose between Dec. 14, 2020, and July 20, 2021. They identified cases of myocarditis after vaccination using reports from clinicians in the Kaiser Permanente network’s Regional Immunization Practice Committee and hospitalization data within 10 days of vaccination.

Incidence of myocarditis

Among the study cohort, about half of patients received at least one dose of the Moderna vaccine and the other half received at least one dose of the Pfizer-BioNTech vaccine. Fifty-four percent of participants were women, and 37.8% identified as Hispanic, 31.2% identified as white, 14.3% identified as Asian and 6.7% identified as Black. The median age was 49 years; 35.7% of participants were younger than 40 years. Overall, 93.5% of participants received two doses of a vaccine.

Lee and colleagues identified 15 cases of confirmed myocarditis among vaccinated men — two after the first vaccine dose and 13 after the second vaccine dose. Specifically, nine white men, four Hispanic men, one Asian man and one man of unknown ethnicity had myocarditis. This resulted in an observed incidence rate of 0.8 cases of myocarditis per 1 million first doses and 5.8 cases per 1 million second doses within 10 days of vaccination. The men who had confirmed myocarditis after vaccination had no history of a prior cardiac disease, according to Lee and colleagues. All were hospitalized, and all tested negative for SARS-CoV-2. Fourteen out of the 15 participants experienced chest pain within 1 to 5 days of vaccination, but all symptoms resolved with conservative management.

“This vaccinated cohort is unique in its racial and ethnic diversity and in receiving care at community hospitals with treatment reflective of real-world practice,” Lee and colleagues wrote.

Weighing risk

Lee stressed that “the benefits of the COVID-19 vaccine greatly outweigh the risks.”

“Getting the COVID-19 vaccine is one of the best things people can do to protect themselves from getting or spreading COVID-19,” she said.

In a related editorial, Vinay Guduguntla, MD, an internal medicine resident at the University of California, San Francisco, and Mitchell H. Katz, MD, president and CEO of NYC Health + Hospitals, emphasized that the minimal risk for rare myocarditis cases “is small when weighted with the morbidity and mortality of COVID-19 infections.”

They called the incidents of vaccination-related myocarditis “rare” and a “mostly mild adverse event.” Meanwhile, they noted that a COVID-19 infection causes signs of myocardial injury in 28% of hospitalized patients.

“Randomized clinical trials show that COVID-19 mRNA vaccines represent a safe and effective method of preventing infection; the identification of rare myocarditis does not change clinical decision-making,” Guduguntla and Katz wrote.


Simone A, et al. JAMA Intern. 2021;doi:10.1001/jamainternmed.2021.5511.

Guduguntla V, Katz MH. JAMA Intern. 2021;doi:10.1001/jamainternmed.2021.5634.