evidence: Whereas myocarditis is a specific form of cardiac inflammation, pericarditis reflects inflammation localized to the pericardium, and the occurrence of cardiac arrhythmias, although associated with both, is not a specific indictor of cardiac inflammation. Thus, neither pericarditis nor any category of cardiac arrhythmia were associated specifically with COVID-19 vaccination10,11,12. Myocarditis is underdiagnosed in practice13, with clinical bias being directed towards myocardial ischemia or infarction. Thus, our use of diagnostic codes for myocarditis from routine data suggest that the ascertainment of cardiac inflammation after COVID-19 vaccination is likely to be under- rather than over-represented14,15. [...] Witberg, G. et al. Myocarditis after Covid-19 vaccination in a large health care organization. N. Engl. J. Med. https://doi.org/10.1056/NEJMoa2110737 (2021).