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

When some people have their large bowel removed, a surgeon can make a "pouch" from part of the small bowel to connect it to the back passage (anus). Pouchitis is when the pouch becomes inflamed (swollen) or infected. The main aim of this study is to find out if vedolizumab improves pouchitis symptoms and pouch inflammation. Other aims include to find out if vedolizumab is well tolerated and if it causes any medical problems (adverse events or side effects) and to look for any changes in the well-being of participants during their treatment with vedolizumab. This study consists of two parts: Part 1 includes the induction and maintenance periods, and Part 2 includes the continued maintenance period. Participants will receive up to 12 infusions of vedolizumab. In Part 1 of the study, first 3 infusions are in first 6 weeks (Day 1, Week 2 and Week 6). Participants who are getting benefit may continue with the treatment for up to 7.5 months (30 weeks) in the maintenance period for Part 1. After completing treatment with vedolizumab in Part 1, participants will visit their clinic for a health check at Week 34. Participants who show clinical response at Week 34 will continue to Part 2, receiving vedolizumab every 8 weeks for an additional 40 weeks, starting at Week 38 and ending with the last dose being at Week 78. Final efficacy assessments, including a pouchoscopy will be performed at Week 82.