Myocarditis after COVID-19 mRNA vaccination in Norway: a nationwide validation study

Background Myocarditis is a potentially severe adverse event after COVID-19 messenger RNA (mRNA) vaccination. Validation of reported cases is essential. We aimed to determine the occurrence, clinical characteristics and short-term outcomes of vaccine-associated myocarditis (VAM) in Norway. Methods In this nationwide, population-based validation study, we used national health registry data and hospital electronic medical records from 27 December 2020 to 30 April 2022. We identified all Norwegian residents who received at least one dose of BNT162b2 or mRNA-1273. By cross-linking registries, we identified myocarditis within 90 days after vaccination. Diagnoses were validated through individual chart review using Brighton Collaboration criteria. VAM was defined as myocarditis without a more likely alternative cause. Results Among 4.1 million vaccinated individuals who received 10.9 million doses, we identified 367 potential myocarditis cases. Of 349 cases reviewed, 177 (51%) were validated as VAM, corresponding to 4.5 cases per 100 000 vaccinated individuals. In total, 110 (62%) cases occurred after the second dose. Of validated cases, 139 (79%) occurred in men. Median age was 30 (IQR 24–50) years for men and 54 (IQR 32–65) years for women. Three (2%) cases were under 18 years. The median hospital stay was 4 (IQR 3–5) days, and the median ejection fraction was 55% (IQR 53%–60%). Seven (4%) patients required intensive care and two (1%) older patients died. No patient required mechanical circulatory support or heart transplantation. Conclusions VAM occurred in 4.5 per 100 000 vaccinated individuals, based on validation of about half of registry-identified myocarditis cases. The acute clinical course was generally mild. National surveillance and systematic validation are essential for reliable estimates of vaccine-associated adverse events. Trial registration number NCT05610423 .