RecruitingPhase 3Drug trialA Study of Vedolizumab in Children and Teenagers With Ulcerative Colitis or Crohn's DiseaseThe main aim of this study is to learn how the body of a child or teenager with moderately to severely active ulcerative colitis (UC) or Crohn's disease (CD) processes vedolizumab (pharmacokinetics) given just under the skin subcutaneously (SC). The participants will be treated with vedolizumab for up to 34 weeks. During the study, participants will visit their study clinic several times.