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 recipients

Among 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. Among all participants aged 16 to 24, myocarditis rates were 18.8 and 4.4 per 100,000 person-years for males and females, respectively.

Adjusted IRRs in homologously vaccinated males aged 16 to 24 were 5.31 (95% CI, 3.68 to 7.68) after a second dose of Pfizer and 13.83 (95% CI, 8.08 to 23.68) for Moderna. An estimated 5.55 (95% CI, 3.70 to 7.39) excess myocarditis cases per 100,000 vaccines occurred after the second dose of Pfizer, compared with 18.39 (95% CI, 9.05 to 27.72) after Moderna. Pericarditis rates were similar.

The study authors concluded, “Our data are compatible with 4 to 7 excess events within 28 days per 100,000 vaccines after a second dose of [Pfizer], and 9 to 28 excess events within 28 days per 100,000 vaccines after a second dose of [Moderna]. The risk of myocarditis associated with vaccination against SARS-CoV-2 must be balanced against the benefits of these vaccines.”

Benefits outweigh low risk

In an editorial, JAMA Cardiology Editor Ann Marie Navar, MD, PhD, of the University of Texas, and Deputy Editor Robert Bonow, MD, of Northwestern University, said that the risk of COVID vaccine-related myocarditis is low and outweighed by the benefits of vaccination.

“At the individual level, immunization prevents not only COVID-19–related myocarditis but also severe disease, hospitalization, long-term complications after COVID-19 infection, and death,” they wrote.

“At the population level, immunization helps to decrease community spread, decrease the chances of new variants emerging, protect people who are immunocompromised, and ensure how the health care system can continue to provide for our communities.”

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