Self-report. Myocarditis is a serious but uncommon side effect after vaccination with SARS-CoV-2 mRNA-based vaccines. In population-based registration studies, the risk of myocarditis was shown to be higher in younger men and after the second dose, and higher for mRNA-1273 (Spikevax) compared to BNT162b2 (Comirnaty). [1]. In these enrollment studies, outcome was defined as a discharge diagnosis from inpatient care coded as myocarditis. It is crucial to establish the validity of these diagnoses in order to correctly identify any association between vaccination against SARS-CoV-2 and the risk of developing myocarditis.

To assess the validity of myocarditis coding in the patient registry, the Medical Products Agency reviewed the medical records of all myocarditis cases that occurred after vaccination against SARS-CoV-2 mRNA in 2021, as well as a random selection of myocarditis cases in 2021 that It did not occur after vaccination [2]. A total of 342 cases were reviewed. The review shows that the diagnosis of myocarditis in 327 cases was rated as correct (definite, probable or possible), giving a positive predictive value of 95.6 percent (327 of 342). The Brighton Collaboration’s myocarditis diagnostic criteria were used for evaluation. To evaluate the extent to which potential misclassification affected previously described risk estimates for myocarditis, previously published data were reanalyzed using the new case definition. Reclassification had very little effect on myocarditis risk estimates [1, 2].

In order to identify potential differences in evaluation between reviewers, a blind selection was made of 51 journals that underwent a second evaluation by a reviewer other than the original reviewer. None of the 30 randomly selected cases initially classified as “clear or probable myocarditis” changed after the second review, and of the 15 cases initially classified as “not myocarditis” or where information was deemed sufficient Incomplete, 7 cases were evaluated after the second review as “probable or likely myocarditis.” The difference in assessment between examiners was mainly due to different interpretations of the ECG.

Overall, review of the medical record shows high validity of the diagnosis of myocarditis in the patient registry. Agreement between reviewers was high. Reclassification of cases had very little effect on estimates of the risk of myocarditis after vaccination against SARS-CoV-2.