The purpose of this study is to assess plasma bimekizumab concentrations following subcutaneous (sc) bimekizumab administration.
Study participant must be 2 to <18 years of age inclusive, at the Baseline Visit.
Study participants who have confirmed diagnosis of enthesitis-related arthritis (ERA; including juvenile-onset ankylosing spondylitis (JAS)) and/or juvenile psoriatic arthritis (JPsA) according to the juvenile-International League of Associations for Rheumatology (JIA-ILAR) classification criteria of at least 3 months duration prior to the Screening Visit.
Study participants who have active disease (ERA [including JAS] and/or JPsA) defined as having at least 3 active joints, each of which needs to be included in the joints assessed in the JADAS27, and for ERA at least 1 site of enthesitis at Baseline or documented by history.
Study participants with inadequate response (at least 1 month) or intolerance to at least 1 nonsteroidal anti-inflammatory drug (NSAID).
Study participants fulfilling any International League of Associations for Rheumatology (ILAR) diagnostic juvenile idiopathic arthritis (JIA) category other than enthesitis-related arthritis (ERA; including juvenile-onset ankylosing spondylitis (JAS)) and/or juvenile psoriatic arthritis (JPsA).
Study participant has history of inflammatory bowel disease (IBD) or signs/symptoms suggestive of IBD.
Study participant has active uncontrolled uveitis.
Study participant has history of active tuberculosis (TB) unless successfully treated, latent TB unless prophylactically treated.