A Study of Vedolizumab in Children and Teenagers With Ulcerative Colitis or Crohn's Disease

Inclusion Criteria: 1. The participant weighs ≥10 kg at the time of screening and enrollment into the study. 2. Participants with UC or CD diagnosed at least 1 month before screening. Participants with moderately to severely active disease defined as: * Participants with UC: a modified Mayo score of 5 to 9 (sum of Mayo endoscopic subscore, stool frequency subscore, and rectal bleeding subscore) with a Mayo endoscopic subscore of ≥2 (with the presence of mucosal friability excluding an endoscopic subscore of 1 and mandating a score of at least 2). (The results of screening endoscopy should be applied.) * Participants with CD: a pediatric Crohn's disease activity index (PCDAI) \>30 and a simple endoscopic score for Crohn's disease (SES-CD) \>6 (or an SES-CD ≥4 if disease is confined to terminal ileum) at screening endoscopy. 3. Participants who have failed, lost response to, or been intolerant to treatment with at least 1 of the following agents: corticosteroids, immunomodulators (for example, azathioprine \[AZA\], 6-mercaptopurine \[6-MP\], methotrexate \[MTX\]), and/or tumor necrosis factor (TNF)-α antagonist therapy (for example, infliximab, adalimumab). 4. Participants with evidence of UC extending proximal to the rectum (that is, not limited to proctitis), at a minimum. 5. Participants with extensive colitis or pancolitis of \>8 years' duration or left-sided colitis of \>12 years' duration must have documented evidence of a negative surveillance colonoscopy within 12 months before screening. 6. Participants with vaccinations that are up-to-date based on the countrywide accepted schedule of childhood vaccines. Exclusion Criteria: 1. Participants who have had previous exposure to approved or investigational anti-integrins, including but not limited to, natalizumab, efalizumab, etrolizumab, or abrilumab (AMG 181); or mucosal addressin cell adhesion molecule-1 (MAdCAM-1) antagonists (ontamalimab), or rituximab. 2. Participants who have had prior exposure to vedolizumab. 3. Participants with hypersensitivity or allergies to vedolizumab or any of its excipients. 4. Participants with active cerebral/meningeal disease, signs/symptoms or history of progressive multifocal leukoencephalopathy (PML) or any other major neurological disorders. 5. The participant has received any live vaccinations within 30 days before first dose of study drug. 6. Participants who currently require surgical intervention or are anticipated to require surgical intervention for UC or CD during this study. 7. Participants who have had subtotal or total colectomy or have a jejunostomy, ileostomy, colostomy, ileo-anal pouch, known fixed stenosis of the intestine, short bowel syndrome, or \>3 small intestine resections. 8. Participants with a current diagnosis of indeterminate colitis. 9. Participants with clinical features suggesting monogenic very early-onset inflammatory bowel disease (IBD). 10. Participants with active or latent tuberculosis (TB). 11. Participants with evidence of positive hepatitis B surface antigen (HBsAg) or hepatitis B core antibody (HBcAb). Hepatitis B virus (HBV) immune subjects (that is, HBsAg negative and hepatitis B surface antibody \[anti-HBs\]-positive) may, however, be included. 12. The participant has any identified congenital or acquired immunodeficiency (for example, common variable immunodeficiency, human immunodeficiency virus \[HIV\] infection, organ transplantation). 13. Participants with positive stool studies for ova and/or parasites or stool culture at screening visit. 14. Participants with positive Clostridioides difficile (C difficile) stool test at screening visit.