AbstractPurposeThe risk of myopericarditis following COVID-19 booster vaccination has not been extensively evaluated. We provide a timely case ascertainment of myocarditis following COVID-19 booster vaccine in individuals age 18-39 years from an integrated health system.MethodsWe studied a cohort of 65,785 Kaiser Permanente (KP) Northwest Health Plan members aged 18-39 years who received a COVID-19 vaccine booster at least 5 months following completion of the primary series. We identified cases of myopericarditis by searching the electronic health record for the National Center for Health Statistics (NCHS) text label for ‘myocarditis’ or ‘pericarditis’ diagnosis codes in all inpatient and outpatient encounters through January 18th 2022. The cohort was followed for 21 days after their booster. We excluded anyone with a documented diagnosis of myocarditis or pericarditis before their first COVID-19 vaccination. Two physicians independently reviewed the identified patient records and applied the CDC myocarditis and pericarditis surveillance case definition to classify records as confirmed, probable or excluded based on the prior published definition.ResultsOur method identified 6 patients who met the confirmed or probable CDC case definition for acute myocarditis or pericarditis within 21 days of COVID-19 booster dose among 65,785 eligible members. Four cases occurred in 27,253 men. Overall, we estimated 9.1 cases (exact 95% CI 3.4 to 19.9) of post-booster myopericarditis per 100,000 booster doses given. In men, we estimated 14.7 cases (exact 95% CI 4.0 to 37.6) per 100,000 booster doses given.ConclusionWe identified a rate of 9.1 cases of myopericarditis per 100,000 COVID-19 booster doses which is higher than prior estimates reported by the Vaccine Adverse Event Reporting System (VAERS). Myopericarditis occurs following COVID-19 booster vaccine and may be underreported by current surveillance methods. High sensitivity of these case estimates is essential when modeling risk and benefit for sequential COVID-19 vaccinations for the general population.