Booster Vaccination With SARS-CoV-2 mRNA Vaccines and Myocarditis

Quick Takes

  • A booster dose of vaccine was associated with increased risk of myocarditis within 28 days of vaccination in 12- to 39-year-old males with incident rates of 0.86 per 100,000 vaccinated individuals for the BioNTech/Pfizer vaccine and 1.95 per 100,000 for the Moderna vaccine.
  • No deaths occurred within 30 days of vaccine-related myocarditis cases in this study.

Study Questions:

What is the rate of myocarditis after booster vaccination with severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) messenger RNA (mRNA) vaccines?


A multinational cohort study was performed using nationwide registries from Denmark, Finland, Norway, and Sweden. Adjusted incident rate ratios (IRRs) were calculated and country-specific results were combined in meta-analyses.


A total of 8.9 million residents were followed for 12,271,861 person-years, and 1,533 cases of myocarditis were identified. There was an increased incidence rate of myocarditis for both vaccines during the 28-day acute risk period after the third vaccine dose as compared with the post-acute risk period ≥28 days after the second dose (BNT162b2 [BioNTech and Pfizer] IRR, 2.08 [95% confidence interval (CI), 1.31-3.33], mRNA-1273 [Moderna] IRR, 8.89 [95% CI, 2.26-35.03]). For females, the IRR was only estimable for the BNT162b2 vaccine (IRR, 3.99; 95% CI, 0.41-38.64). The absolute risk for males following the third dose was 0.85 (95% CI, 0.53-1.32) myocarditis events per 100,000 individuals vaccinated with the BNT162b2 vaccine and 1.95 (95% CI, 0.53-4.99) events per 100,000 individuals vaccinated with the mRNA-1273 vaccine. For females with the BNT162b2 vaccine, the absolute risk was 0.15 (95% CI, 0.04-0.39) events per 100,000 individuals vaccinated. No deaths occurred within 30 days of vaccine-related myocarditis cases.


The authors conclude that while there is increased risk of myocarditis associated with a booster dose in adolescents and young adults, the absolute risk is low.


Soon after widespread immunization programs with SARS-CoV-2 mRNA vaccines, myocarditis was identified as a rare but concerning adverse event after vaccination. This study evaluated the risk of myocarditis after booster doses with both the Moderna and BioNTech/Pfizer vaccines and found an increased risk in adolescents and young adults, particularly in males. Although not designed to show differences in myocarditis risk between the different vaccines, this study did suggest a higher rate with the Moderna vaccine, a trend also seen with primary vaccine series. It is possible that the lower dose of vaccine with the Moderna booster may attenuate this effect.

While healthy young males are at minimal risk for serious complications from COVID-19, they are at the highest risk for vaccine-associated myocarditis. This makes a detailed risk/benefit conversation with patients and their families an important part of counseling. No mortality was attributed to vaccine-associated myocarditis in this study. Because of serious potential complications of COVID-19 disease including myocarditis, the Centers for Disease Control and Prevention and the Advisory Committee on Vaccination Practices recommend a complete vaccine series for all patient populations without a preference for vaccine type.

Clinical Topics: COVID-19 Hub, Heart Failure and Cardiomyopathies, Prevention

Keywords: COVID-19 Vaccines, Myocarditis, SARS-CoV-2

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