Study participants will be screened during the platform study and randomly assigned to receive mirikizumab or another intervention. The purpose of the mirikizumab study is to evaluate efficacy, safety, tolerability, and how well mirikizumab absorbs into the body of pediatric participants with Crohn's disease.

Study periods for the intervention-specific appendix (ISA) will be as follows:

• A 12-week induction period
• A maintenance period from Week 12 to Week 52, and
• A safety follow-up period up to 16 weeks.

The study will last about 74 weeks and may include up to 19 visits.

Participants screened in the MACARONI-23 Platform study could be randomized to mirikizumab to participate in this intervention specific arm of the study.

Inclusion Criteria:

• Participants must have a diagnosis of CD or fistulizing CD, with active colitis, ileitis, or ileocolitis, confirmed at any time in the past by clinical, endoscopic, and histologic criteria.
• Participants have moderately to severely active CD (as defined by a baseline PCDAI score ≥30).
• Participants must have endoscopy with evidence of active CD defined as SES-CD score ≥6 (or ≥4 for participants with isolated ileal disease) within 1 month of receiving study intervention at Week 0.
• Participants must have a documented history of inadequate response, loss of response or intolerance to at least one medication used to treat CD, which may include immunomodulators, oral or IV corticosteroids, a biologic therapy or a JAK inhibitor.

Exclusion Criteria:

• Participants must not have complications of CD such as symptomatic strictures or stenosis, short gut syndrome, or any other manifestations that might be anticipated to require surgery.
• Participants must not have an abscess.
• Participants must not have any kind of bowel resection within 26 weeks or any other intra-abdominal surgery within 12 weeks of baseline.

Anonymized individual patient level data will be provided in a secure access environment upon approval of a research proposal and a signed data sharing agreement.

Data are available 6 months after the primary publication and approval of the indication studied in the US and European Union (EU), whichever is later. Data will be indefinitely available for requesting.

A research proposal must be approved by an independent review panel and researchers must sign a data sharing agreement.