Unmasking Eosinophilic Granulomatosis with Polyangiitis: A Case of Rapid-Onset Myositis Following COVID-19 Booster in an Eosinophilic Asthma Patient

Eosinophilic Granulomatosis with Polyangiitis (EGPA) is a rare Anti-neutrophil cytoplasm antibodies (ANCA)-associated vasculitis with multi-organ involvement and eosinophilia. We present a 61-year-old male with a history of eosinophilic asthma who developed progressive weakness and muscle aches six weeks after his second COVID-19 booster. Four to six weeks post vaccination, he developed myal gias and weakness, especially in proximal muscles, leading to a severe decline in function. Lab results showed leukocytosis (29.54 cells/mm³); 54% eosinophils; and elevated erythrocyte sedimentation rate (ESR), C-reactive protein, and creatine kinase levels at 13 736 u/L. Anti-myeloperoxidase antibodies were positive, while PR-3, C-ANCA, and P-ANCA were negative. Magnetic resonance imaging of the lower extremities showed intramuscular edema. Muscle biopsies confirmed EGPA. Diagnosed per American College of Rheumatology (ACR)/ European Alliance of Associations for Rheumatology (EULAR) criteria, he received pulse dose IV methylprednisolone with significant improvement. He was discharged on 60 mg prednisone and started on mepolizumab (300 mg subcutaneous once every 4 weeks). Eosinophilic granulomatosis with polyangiitis progresses through stages: asthma, eosinophilia with organ involvement, and vasculitis. This case highlights a unique presentation of rapid myositis without significant involvement of any other organs and vasculitis post-mRNA vaccination. While environmental factors may trigger EGPA, the role of COVID-19 vaccines remains hypothesis-generating. This case underscores the importance of post-market surveillance for rare events, contributing to the understanding of vaccine-related rheumatic disease triggers Cite this article as: Memdani A, Busa V, Avula S, Ford A, Nesheiwat J. Unmasking Eosinophilic granulomatosis with polyangiitis: a case of rapid-onset myositis following COVID-19 booster in an eosinophilic asthma patient. Eur J Rheumatol. 2025, 13(1), 0016, doi:10.5152/eurjrheum.2026.25016.