A Study of Vedolizumab in Children and Teenagers With Moderate to Severe Ulcerative Colitis (UC)

The drug being tested in this study is called vedolizumab. Vedolizumab is being tested to treat pediatric participants who have moderately to severely active UC. Participants to be enrolled must have failed response to, lost response to, or been intolerant to at least 1 of the current standard of care (SOC) induction and maintenance therapies for UC including immunomodulators (eg, azathioprine \[AZA\], 6-mercaptopurine \[6-MP\], methotrexate \[MTX\]), and tumor necrosis factor-alpha (TNF-α) antagonists (eg, infliximab, adalimumab). The study will enroll approximately 120 patients. During the Induction Period participants will receive 3 doses of vedolizumab IV infusion at Day 1, Week 2, and Week 6 based on their weight at Baseline: * Participants ≥30 kg, Vedolizumab 300 mg * Participants \>15 to \<30 kg, Vedolizumab 200 mg * Participants 10 to 15 kg, Vedolizumab 150 mg At Week 14, participants who achieve clinical response will be randomly assigned (by chance, like flipping a coin) in a 1:1 ratio to one of the 2 double-blind dose groups (high dose and low dose), stratified by previous exposure/failure to TNF-α antagonists therapy or naive to TNF-α antagonists therapy, and weight. Participants will receive vedolizumab IV infusions every 8 weeks (Q8W) up to Week 46 during the Maintenance Period as follows: * Participants ≥30 kg, Vedolizumab 300 mg (High dose) or 150 mg (Low dose) * Participants \>15 to \<30 kg, Vedolizumab 200 mg (High dose) or 100 mg (Low dose) * Participants 10 to 15 kg, Vedolizumab 150 mg (High dose) or 100 mg (Low dose) The dose will remain blinded to the participant and study doctor and staff during the study (unless there is an urgent medical need). All participants will be administered vedolizumab via IV infusion. In participants who demonstrate lack of maintenance of clinical response during the Maintenance Period the dose will be escalated in a blinded fashion based on the weight at the time of the worsening of disease. In addition one-time rescue therapy with corticosteroids is allowed during Maintenance period. This multi-center trial will be conducted worldwide. After the Week 54 visit, participants who are younger than 18 years may be eligible to continue receiving vedolizumab in extension study MLN0002-3029 (NCT05442567). Participants who do not maintain corticosteroid-free clinical response at Week 54 or who discontinue study drug at any time during the induction or maintenance periods of this study will undergo an end of study (EOS) or early termination (ET) visit, as well as a safety visit 18 weeks after the last dose of vedolizumab, in addition these participants would enter study MLN0002-3029 for an observational long-term follow-up (LTFU) period of 2 years after the last dose of study drug in the current study. During the LTFU period, data will be collected either by clinic visit OR, if site attendance is not feasible, by phone call every 6 months.