Abstract 11370: Acute Myocarditis Following a Third Dose of COVID-19 mRNA Vaccination in Adults

Introduction: The COVID-19 mRNA vaccines are effective in reducing COVID-19-related severe disease and death. Waning vaccine effectiveness has prompted the recommendation to administer additional (booster) doses. With additional doses of COVID-19 mRNA vaccines being recommended, it is essential to monitor its safety. Myocarditis has been reported following the second dose of COVID-19 mRNA vaccination. Whether administration of additional doses of COVID-19 vaccines further increases the risk of myocarditis is not known. This study aimed to evaluate whether a third dose of COVID-19 mRNA vaccine was associated with an increased risk of myocarditis in a large population-based cohort in California. Methods: We included individuals who received one to three doses of BNT162b2 (Pfizer) or mRNA-1273 (Moderna) mRNA vaccine between 12/14/2020 and 2/18/2022 in an integrated healthcare delivery system in the United States. Hospitalization for myocarditis within 21 days of vaccine administration was identified using electronic medical records. Results: Of 3,076,660 KPSC members who received at least one dose of COVID-19 mRNA vaccines, 2,916,739 (94.5%) received at least two doses, and 1,146,254 (47.0%) received three doses. The IRR for myocarditis was 0.86 (95% CI 0.31-1.93) for the first dose, 4.87 (95% CI 3.14-7.37) for the second dose, and 2.61 (1.13-5.29) for the third dose. The majority of myocarditis cases following the second and third dose occurred within seven days of vaccination. Conclusion: Myocarditis was a rare but serious event observed after the second or third dose of vaccination. Most cases presented within seven days of vaccination. The incidence of myocarditis following the third dose did not appear to be significantly higher than that observed after the second dose.