This Phase III, randomized, double-blind, multicenter, induction and maintenance study will evaluate the safety and efficacy of Afimkibart (RO7790121) in pediatric participants with moderate to severe active ulcerative colitis (UC).

Inclusion Criteria:

• Bodyweight >= 10 kilogram (kg)
• Confirmed diagnosis of UC
• Demonstrated intolerance or inadequate response (IR) to one or more of the following categories of drugs: systemic corticosteroids, immunomodulators, and/or biologic therapies as outlined in the protocol

Exclusion Criteria:

• Monogenic disorder pertaining to infant onset inflammatory bowel disease (IBD)
• Current diagnosis of Crohn's disease (CD), abdominal/intrabdominal/perianal fistula and/or abscess, indeterminant colitis, IBD-unclassified, microscopic colitis, ischemic colitis, infectious colitis, radiation colitis, or active diverticular disease
• Presence of an ostomy or ileoanal pouch
• Current diagnosis or suspicion of primary sclerosing cholangitis
• Any major surgery within 6 weeks prior to screening or a major planned surgery during the study
• Active tuberculosis (TB) infection suggested by positive TB testing, clinical symptoms, and/or chest imaging (X-ray or CT)

For eligible studies, qualified researchers may request access to individual patient level clinical data. See Roche's commitment to transparency of clinical study information here: https://go.roche.com/data\_sharing