A Study Evaluating the Effects of Filgotinib in Children and Teenagers With Ulcerative Colitis

Inclusion Criteria: * Subject must have a minimum body weight (BW) of 15 kg. * Subject: * has documented diagnosis of UC with a minimum duration of 3 months, * has mMCS of 5 to 9, and an MCS endoscopic score \>=2, rectal bleeding \>=1, and stool frequency \>=1, * has had an inadequate response, loss of response, intolerance, or has medical contraindications to corticosteroids, immunosuppressants, and/or biologic therapy. This includes subjects who depend on corticosteroids to control their symptoms and who experience worsening of their disease when attempting to wean off corticosteroids. Exclusion Criteria: * Subject has a diagnosis of inflammatory bowel disease -unclassified or indeterminate colitis, isolated proctitis, or toxic megacolon. * Subject has an active infection. * Subject with a history of complicated herpes zoster infection (with multi-dermatomal, disseminated, ophthalmic, or central nervous system involvement). * Currently on any therapy for chronic infection (such as pneumocystis, cytomegalovirus, herpes simplex, herpes zoster, or atypical mycobacteria). * Subject has a history of colectomy or extensive small bowel resection. * Subject with psychological or cognitive difficulties that might interfere with study participation. * Subject has any previous exposure to a Janus kinase inhibitor or medication with a similar mode of action (e.g. tofacitinib, baricitinib, upadacitinib). * Female subject is pregnant or breast feeding or intending to become pregnant or breastfeed during the study.