Abstract 9395: Natural History of Myocardial Injury Following Covid-19 Vaccine Associated Myocarditis

Introduction: Acute myocarditis is a rare complication of mRNA-based COVID-19 vaccination. The natural history and clinical sequalae of this complication are of critical importance to estimate the impact on population health. Objective: To study the impact of COVID-19 vaccine associated myocarditis on convalescent myocardial health, symptoms, and clinical outcomes. Methods: This study was performed at two tertiary care hospitals. Baseline and convalescent (≥3 month) CMR imaging was performed for 20 consecutive patients meeting Updated Lake Louise Criteria for acute myocarditis within 10 days of mRNA-based vaccination and with no evidence of active viral infection. Global and segmental CMR-based markers of myocardial injury were studied at baseline and convalescence, inclusive of chamber volumetry, LV ejection fraction (LVEF), LV mass, T1 and T2 mapping, late gadolinium enhancement (LGE) imaging, and extracellular volume (ECV) fraction. Major clinical outcomes were captured. Results: The median age was 23.1 years (range 18-39 years) with 17 (85%) being male. Convalescent CMR evaluations were performed at a median (IQR) of 3.7 (3.3-6.2) months. Five patients (25%) had a baseline LVEF <50%. All patients showed abnormal LGE inclusive of a sub-epicardial pattern with a mean LGE burden of 8.6+/-5.3% LV mass using a >5SD of reference threshold. At follow-up, a mean 3% absolute improvement in LVEF was observed, accompanied by a 7% reduction in LVEDV and 5% reduction in LV mass (all p<0.015). Global LGE volume reduced by 66% (p<0.001). Absolute reductions in global T2, native T1, and ECV of 2.1msec, 58msec, and 2.9%, respectively were documented (all p≤0.001). All patients with LVEF <50% at baseline recovered above this threshold. However, 18 (90%) patients showed persistence of LGE at follow-up with 15% having a burden >5% of the LV mass. No patient experienced major clinical outcomes. Conclusion: COVID-19 mRNA vaccine associated myocarditis shows rapid improvement in CMR-based markers of tissue edema, contractile function, and LGE, and was not associated with adverse events at intermediate range follow-up. However, convalescent fibrosis was commonly observed, justifying ongoing need for clinical surveillance in this population.