In the cohort of 23.1 million residents across four Nordic countries, 74% had received two doses (17,129,982 people) and 7% (1,681,930 people) one dose of mRNA COVID-19 vaccine by the end of the study period. Out of 18,811,912 vaccinated people, 1077 cases of myocarditis and 1149 cases of pericarditis with onset within 28 days of vaccination were identified.
The study SARS-CoV-2 vaccination and myocarditis in a Nordic cohort study of 23 million residents published in JAMA Cardiology, online ahead of print, found both first and second doses of mRNA COVID-19 vaccine were associated with an increased risk of myocarditis or pericarditis. For young males aged 16–24 years who received two doses of the same vaccine, the findings were compatible with 4–7 excess cases in 28 days per 100,000 vaccinees after BNT162b2 (Pfizer/BioNTech) and 9–28 excess cases per 100,000 vaccinees after mRNA-1273 (Moderna). These data are consistent with reports and data from the U.S., Israel, France, England, and Canada. Cases of myocarditis associated with COVID-19 disease were also identified with 3.26 excess cases per 100,000 males with a positive test, and 1.37 excess cases in young males aged 16–24 years with a positive test.
This risk of myocarditis or pericarditis should be balanced against the benefits of protecting against severe COVID-19 disease.