LAUSR

A female 17‐year‐old diagnosed with seronegative spondyloarthritis (SpA) following the first jab of the BioNTech162b2 (BNT162b2) vaccine presented with recurrent swelling and painful knee accompanied by posterior heel tenderness over the past 1.5 months. Laboratory investigations revealed elevated serum erythrocyte sedimentation rate and C‐reactive protein. Synovial aspiration yielded level 3 crystal‐free, aseptic and inflammatory effusion. She tested positive for the human leukocyte antigen‐B27 and was diagnosed with peripheral SpA. She received daily celecoxib (400 mg), methylprednisolone (8 mg), and sulfasalazine (2 g), but the effect was limited. Nonetheless, her symptoms improved significantly with weekly subcutaneous etanercept administration (50 mg). Four weeks later, her arthritis was completely resolved. To our knowledge, this is the first case report of newly diagnosed seronegative peripheral SpA in an autoimmunity‐disease‐free individual following messenger RNA BNT coronavirus disease 2019 vaccination.