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

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.