A new study of 23.1 million Scandinavians suggests that the risk of myocarditis after mRNA COVID-19 vaccination is low but highest in 16- to 24-year-old males after the second dose.
The results were published yesterday in JAMA Cardiology.Researchers from the Norwegian Institute of Public Health studied the incidence of myocarditis (inflammation of the heart muscle) and pericarditis (swelling of the tissues around the heart) among residents of Denmark, Finland, Norway, and Sweden aged 12 and older before vaccination or 28 days after the first or second vaccine doses.Eighty-one percent of participants were vaccinated by study end.
All were followed from Dec 27, 2020, to Oct 5, 2021.Under 30 cases per 100,000 vaccine recipientsAmong all participants, 1,077 and 1,149 developed myocarditis and pericarditis, respectively, before or after vaccination.One-hundred-five participants developed myocarditis after the first dose of the Pfizer/BioNTech vaccine, and 115 did so after the second.
Among Moderna recipients, 15 developed myocarditis after the first dose, as did 60 after the second.Of recipients of two doses of the same vaccine (homologous vaccination), the second dose was tied to a 75% elevated risk of myocarditis for Pfizer (adjusted incidence rate ratio [IRR], 1.75; 95% confidence interval [CI], 1.43 to 2.14) and a more than sixfold increased risk for Moderna (IRR, 6.57; 95% CI, 4.64 to 9.28).The researchers noted 9.7 myocarditis cases per 100,000 person-years for unvaccinated males and 4.3 per 100,000 for females.