A Study to Evaluate the Pharmacokinetics, Safety and Efficacy of Afimkibart (RO7790121) in Children With Moderately to Severely Active Ulcerative Colitis

Inclusion Criteria:

* Bodyweight \>= 10 kilogram (kg)
* Confirmed diagnosis of UC
* Moderately to severely active UC, defined as modified Mayo Score (mMS) or 5 to 9 points, including endoscopic score of 2 or 3, confirmed by centrally read endoscopy (flexible sigmoidoscopy or colonoscopy)
* 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)