A Study of Vedolizumab in Children With Ulcerative Colitis (UC) or Crohn's Disease (CD)

The study is an extension of two parent studies (MLN0002-3024 \[NCT04779307\] and MLN0002-3025 \[NCT04779320\]). Participants must have participated in one of the previous studies. The purpose of this study is to collect the long-term safety of vedolizumab in children with UC or CD.

This multi-center trial is conducted worldwide. Up to 240 patients would be enrolled from Studies MLN0002-3024 \[participants with UC\] and MLN0002-3025 \[participants with CD\], either in the Treatment Cohort or in the Observational Cohort. Approximately 93 participants who have previously participated either in study MLN0002-3024 or MLN0002-3025, referred to as parent study, are expected to roll over to the MLN0002-3029 study in the treatment cohort. Treatment Cohort: The drug being tested in this study is called vedolizumab, being studied to treat pediatric patients who have UC or CD. Participants eligible for the Treatment Cohort can be administered vedolizumab intravenous (IV) at Week 54 visit of parent study or up to 1 week after Week 54 of the parent study based on the availability of test results needed to assess eligibility of the participant. At this study entry, participants will be administered the same blinded dose of vedolizumab IV that was received at Week 46 in the parent study and will then continue to receive vedolizumab IV at a frequency of once every 8 weeks (Q8W) in the following treatment groups: * Participants 10 to ≤15 kilogram (kg), Vedolizumab 150 milligram (mg) (High dose) * Participants 10 to ≤15 kg, Vedolizumab 100 mg (Low dose) * Participants \>15 to \<30 kg, Vedolizumab 200 mg (High dose) * Participants \>15 to \<30 kg, Vedolizumab 100 mg (Low dose) * Participants ≥30 kg, Vedolizumab 300 mg (High dose) * Participants ≥30 kg, Vedolizumab 150 mg (Low dose) Blinding of dose group assignment of the parent study will continue until the respective parent study is unblinded in order to protect the blinding of the parent study. The overall time to participate in the Treatment Cohort of this study is up to participant withdrawal, or until vedolizumab IV is commercially available for pediatric indication(s) in the participant's country or until other drug access programs become available, or Sponsor's decision for study closure, or for up to approximately 5 years, whichever comes first. Participants who complete or are discontinued from the study for any reason will complete the final safety/end of study (EOS) visit 18 weeks after their last dose of study drug. Observational Cohort: Participants who received at least 1 dose of study drug during parent study and early terminated or are not eligible for the Treatment Cohort of this study after completion of the Week 54 visit of parent study, will be enrolled in the Observational Cohort of this study as part of a long-term follow-up period to assess prespecified safety events of interest and will not receive continued treatment with vedolizumab IV. The overall time to participate in the Observational Cohort is up to approximately 2 years.

Main Inclusion Criteria: For Treatment Cohort: 1. The participant should have completed Study MLN0002-3024 or Study MLN0002-3025 and achieved corticosteroid-free clinical response at Week 54 (and has tapered off of steroids, as applicable, at least 12 weeks before Week 54) as defined by a reduction of partial Mayo score of ≥2 points and ≥25% from baseline for participants with UC, or by a decrease of pediatric Crohn's disease activity index (PCDAI) of ≥15 points for participants with CD and with total PCDAI ≤30. 2. A male participant who is sexually active with a female partner of childbearing potential agrees to use a barrier method of contraception (e.g., condom with or without spermicide) from signing of participant/parental informed consent and/or pediatric assent throughout the duration of the study and for 18 weeks after last dose. The female partner of a male participant should also be advised to use a highly effective method of contraception. 3. A female participant of childbearing potential who is sexually active with a nonsterilized male partner agrees to use a highly effective method of contraception from signing of participant/parental informed consent and/or pediatric assent throughout the duration of the study and 18 weeks after the last dose. For Observational Cohort: 1\. The participant has received at least 1 dose of vedolizumab during Study MLN0002-3024 or Study MLN0002-3025 and early terminated OR completed the Week 54 visit of Study MLN0002-3024 or Study MLN0002-3025 but was not eligible to enroll in the treatment cohort of this study. Main Exclusion Criteria: For Treatment Cohort only: 1. The participant currently requires major surgical intervention for UC or CD (e.g., bowel resection), or is anticipated to require major surgical intervention for UC or CD during the study. 2. The participant has developed any new unstable or uncontrolled cardiovascular, heart failure moderate to severe (New York Class Association III or IV), pulmonary, hepatic, renal, gastrointestinal (GI), genitourinary, hematological, coagulation, immunological, endocrine/metabolic, neurological, or other medical disorder that, in the opinion of the investigator, would confound the study results or compromise participant safety. 3. The participant has other serious comorbidities that will limit their ability to complete the study. 4. The participant is unable to comply with all study assessments. 5. The participant has hypersensitivity or allergies to any of the vedolizumab excipients. 6. The participant is lactating or pregnant.