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 .

Keratoconus is a progressive eye disease that results in thinning of the cornea, leading to visual impairment. Mitral valve prolapse (MVP) is a common disorder affecting around 2–4% of the general population. Previous studies have found an overrepresentation of MVP in individuals with keratoconus, with a prevalence of 38–65%, suggesting a shared underlying mechanism. In this case-control study, patients with keratoconus were enrolled from a quality and research registry. They were examined by a 2D echocardiography to identify if they had MVP, billowing or normal mitral leaflets. Controls were matched from the population-based Trøndelag Health Study. Patients and controls underwent a detailed echocardiographic examination to detect abnormal mitral valves. We included 101 patients (age 33 [IQR 29–40], 75% male) with keratoconus and 101 matched individuals. MVP was found in 2 (2%), while billowing was found in 5 (5%) of keratoconus patients. No significant association was found between keratoconus and the prevalence of MVP or billowing compared to the control group. Moreover, no associations were found between severity of keratoconus with presence of MVP nor with billowing of the mitral valves. We could not confirm the previously reported association between keratoconus and MVP, suggesting that routine screening for MVP in keratoconus patients may not be warranted. However, we cannot rule out the possibility of an association in other gender, age and ethnic groups different than ours.