Myocarditis Following COVID-19 mRNA Vaccine: A Case Series and Incidence Rate Determination.

Background Myocarditis following COVID-19 mRNA vaccines (Pfizer-BioNTech and Moderna) have been increasingly reported. Incidence rates in the general population are lacking, with pericarditis rather than myocarditis diagnostic codes being used to estimate background rates. This comparison is critical to balance the risk of vaccination with the risk of no vaccination. Methods A retrospective case-series was performed utilizing the Mayo Clinic COVID-19 Vaccine Registry. We measured the incidence rate ratio for myocarditis temporally related to COVID-19 mRNA vaccination compared to myocarditis in a comparable population from 2016 through 2020. Clinical characteristics and outcomes of the affected patients was collected. A total of 21 individuals were identified, but ultimately 7 patients met the inclusion criteria for vaccine-associated myocarditis. Results The overall incidence rate ratio (IRR) of COVID-19 related myocarditis was 4.18 (CI95% 1.63, 8.98) which was entirely attributable to an increased IRR among adult males (IRR 6.69, CI95% 2.35, 15.52) compared to females (IRR 1.41, CI95% 0.03, 8.45).All cases occurred within 2 weeks of a dose of the COVID-19 mRNA vaccine with the majority occurring within 3 days (range 1-13 days) following the second dose (6/7 patients, 86%). Overall, cases were mild, and all patients survived. Conclusions Myocarditis is a rare adverse event associated with COVID-19 mRNA vaccines, and in adult males it occurs with significantly higher incidence than the background population rate. Recurrence of myocarditis after a subsequent mRNA vaccine dose is not known at this time.