A Study to Learn About the Safety of Vedolizumab and How Well it Works in Children and Teenagers With Active Chronic Pouchitis

Inclusion Criteria:

1. The participant weighs >=10 kg at the time of screening and first dose.

2. Has active chronic pouchitis, defined by a mPDAI score >=5 assessed using the 3-dayaverage of participant-reported clinical symptoms prior to the screening endoscopy (that is [ie] video pouchoscopy with biopsy) or bowel preparation for the endoscopyand a minimum mPDAI endoscopic subscore of 2 (outside the staple or suture line) andeither:

• >=1 previous episodes of pouchitis within 1 year before the screening visit,with symptoms lasting for at least a total of 4 weeks, treated with >=2 weeksof antibiotic or other prescription therapy (ie, other antibiotics, probiotics,immunomodulators, or anti-tumor necrosis factor [TNFs] within 1 year beforescreening). Or

• Have had an inadequate response with, or lost response to, or be intolerant toantibiotic therapy (ie, requiring maintenance antibiotic therapy taken for >=4weeks immediately before the baseline endoscopy visit or not able to receive orcontinue antibiotic treatment due to intolerance or other contraindication).

3. The participant is aged 2 to 17 years, inclusive, at the time of screening and firstdose.

4. The participant has a history of proctocolectomy and ileal pouch-anal anastomosis (IPAA) as treatment for ulcerative colitis (UC), Crohn's disease (CD), familialadenomatous polyposis (FAP), or other underlying conditions, such as Hirschsprung'sdisease, for which construction of a pouch was medically indicated, completed atleast 1 year before the screening visit.

Exclusion Criteria:

The exclusion criteria are divided into 3 categories: active chronic pouchitis exclusion criteria, infectious disease exclusion criteria, and general exclusion criteria.

Active Pouchitis Exclusion Criteria:

1. Has symptoms believed to be predominantly due to irritable pouch syndrome.

2. Has isolated cuffitis.

3. Is found to have dysplasia at the screening endoscopy.

4. Has mechanical complications of the pouch (for example [e.g.] pouch stricture orpouch fistula).

5. Currently requires or has a planned surgical intervention during the study.

6. Has a diverting stoma. Infectious Disease Exclusion Criteria:

7. Has evidence of an active infection (e.g. sepsis, cytomegalovirus [CMV], orlisteriosis) during screening.

8. Had a clinically significant infection (e.g. pneumonia, pyelonephritis, coronavirusdisease 2019 [COVID-19]) within 35 days before first dose of study drug.

9. Has active or latent tuberculosis (TB), as evidenced by a diagnostic TB testperformed within 3 months of screening or during the screening period that ispositive, as defined by:

• A positive QuantiFERON test or 2 successive indeterminate QuantiFERON tests, or

• A TB skin test reaction >=5 millimeter (mm). NOTE: If participant have receivedBacillus Calmette-Guérin vaccine, then a QuantiFERON TB Gold test should beperformed instead of the TB skin test. NOTE: Participants with documented previously treated TB with a negative QuantiFERONtest can be included in the study.

10. Has evidence of positive hepatitis B surface antigen (HBsAg) or hepatitis B coreantibody (HBcAb). Hepatitis B virus (HBV) immune participants (e.g. HBsAg negativeand hepatitis B antibody positive) may, however, be included. NOTE: If a participant tests negative for HBsAg, but positive for HBcAb, theparticipant would be considered eligible if the absence of HBV DNA is confirmed byHBV DNA polymerase chain reaction reflex testing performed in the centrallaboratory.

11. Has chronic hepatitis C virus (HCV) (ie, positive HCV antibody [HCVAb] and HCVRibonucleic Acid [RNA]). NOTE: Participants who are HCVAb-positive without evidence of HCV RNA may beconsidered eligible (spontaneous viral clearance or previously treated and cured [defined as no evidence of HCV RNA at least 12 weeks before baseline]).

12. Has any identified congenital or acquired immunodeficiency (e.g. common variableimmunodeficiency, HIV infection, organ transplantation).

13. Has positive stool studies for ova and/or parasites or stool culture at screeningvisit.

14. Has positive Clostridium difficile stool test at screening visit. General Exclusion Criteria:

15. Is taking, has taken, or is required to take any excluded medications.

16. Has active cerebral/meningeal disease, signs/symptoms, or history of progressivemultifocal leukoencephalopathy (PML) or any other major neurological disorders,including stroke, multiple sclerosis, brain tumor, or neurodegenerative disease.

17. Has evidence of dysplasia or history of malignancy other than a successfully treatednonmetastatic cutaneous squamous cell or basal cell carcinoma or localized carcinomain situ of the cervix.

18. Has any unstable or uncontrolled cardiovascular, pulmonary, hepatic, renal, GI,genitourinary, hematologic, coagulation, immunological, endocrine/metabolic,neurologic, or other medical disorder that, in the opinion of the investigator,would confound the study results or compromise participant safety.