BACKGROUND AND PURPOSE:

• Simone et al. (JAMA Intern Med, 2021) evaluated post-vaccination acute myocarditis incidence and clinical outcomes in adults who received an mRNA-based COVID-19 vaccine

METHODS:

• Population-based cohort study
  • Kaiser Permanente Southern California (KPSC) 
• Participants
  • ≥18 years
• Exposures
  • Receipt of at least 1 dose of the Pfizer or Moderna COVID-19 vaccine between December 2020 and July 2021
  • Unexposed controls
• Study design
  • Incidence of myocarditis in the vaccinated group was compared to incidence among
    • Cohort of unexposed controls
    • Vaccinated individuals
  • Case identification: Based on reports of hospitalization within 10 days of vaccine administration with a discharge diagnosis of myocarditis
• Primary outcome
  • Cases of post-vaccine myocarditis
  • Outcomes and characteristics of myocarditis cases

RESULTS:

• mRNA COVID-19 vaccine recipients: 2,392,924
  • Median (IQR) age: 49 (34 to 64) years | 54.0% women
  • Completed two doses regimen: 93.5%
• Unexposed cohort: 1,577,741
  • Median (IQR) age: 39 (28 to 53 years) | 49.1% women
• Myocarditis cases
  • After first dose: 2 cases
    • Incidence: 0.8 cases per 1 million first doses
    • Incidence rate ratio (RR) 0.38 (95% CI, 0.05 to 1.40)
  • After second dose: 13 cases
    • Incidence: 5.8 cases per 1 million second doses
    • Incidence RR 2.7 (95% CI, 1.4 to 4.8)
• Case demographics
  • Post-vaccination cases
    • All cases were men
    • Median (IQR) age: 25 (20 to 32) years
    • Cases with prior cardiac disease: 0
    • Received Pfizer: 8 | Received Moderna: 7
  • Unexposed cohort cases
    • Men: 52%
    • Median (IQR) age: 52 (32 to 59) years
• Myocarditis clinical features and outcomes
  • Cases that reported chest pain 1 to 5 days after vaccination: 93%
  • Symptoms resolved with conservative management in all cases
  • No patients required intensive care unit admission or readmission after discharge

CONCLUSION:

• Acute myocarditis following COVID-19 mRNA vaccination was rare and resolved with conservative management
• All cases involved young men, which suggests the risk of myocarditis may be higher in this group

Learn More – Primary Sources:

Acute Myocarditis Following COVID-19 mRNA Vaccination in Adults Aged 18 Years or Older

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