This is a multicenter, double-blind, randomized study to compare the safety, effectiveness and immunogenicity (level and body resistance to drugs) of two biological drugs administered intravenously - vedolizumab (a drug not registered in pediatric patients) and infliximab (used as standard therapy) in pediatric patients with ulcerative colitis aged 6-17 years.

Lab values

• Subjects who were previously treated with any biological drug regardless of indication. 2. Subjects who were previously treated with vedolizumab. 3. Subjects who were previously treated with infliximab 4. Known hypersensitivity to any of the components of study drugs 5. Subjects with active brain/cerebrospinal meninges disease, signs/symptoms or history of progressive multifocal leukoencephalopathy (PML) or any other serious neurological disorders, including stroke, multiple sclerosis, brain cancer or neurodegenerative disease. 6. Subjects who currently require surgical intervention or are expected that they will require surgical intervention due to UC during the study 7. Subjects who have undergone partial or total colectomy or have jejunostomy, ileostomy, colostomy, ileoanalstomy or known permanent bowel stricture 8. Subjects with current diagnosis of unspecified colitis 9. Subjects with clinical features suggesting monogenic very early onset inflammatory bowel disease 10. Presence of active serious infections such as septicemia, abscesses, cytomegalovirus, listeriosis, opportunistic infections. 11. Subjects with moderate or severe heart failure (NYHA class III/IV). 12. individuals with current or past cancer. 13. A subject with other serious co -existing diseases that will limit his/her ability to complete the study 14. Abnormal laboratory tests results: - hemoglobin ≤ 8 g/dl - leukocytes ≤ 2.5 x 10*3/ul. - lymphopenia <0.3 x 10*3/ul. - ALT and/or AST ≥ 3 x upper limit of normal - positive Quantiferon test -signs of active infection confirmed by a positive result in a stool sample for Clostridium difficile and/or stool culture for alarm pathogens -positive HBsAg test -positive anti-HCV antibodies test -positive HIV Ag/Ab test 15. Pregnancy, breastfeeding, or refusal to use effective methods of contraception or sexual abstinence in women of childbearing potential during the study 16. Use of another investigational drug within 6 months before study entry or participation in other studies at the time of screening

• Subjects aged 6 to 17 years. 2. A patient with a moderate or severe UC defined as a minimum of 30 points in the PUCAI, diagnosed at least 1 month before the screening visit 3. Subjects who did not respond to standard treatment, lost their response or did not tolerated treatment of at least one of the following agents: corticosteroids (prednison at a dose of 1mg/kg/d with a maximum dose of 40 mg/d, budesonid MMX 9 mg/d) or immunomodulatory drugs (e.g. AZA, 6-MP, MTX, cyclosporin) - When using 5-ASA, corticosteroids, immunomodulatory drugs dosing should remain stable 2 weeks before the study drug administration 4. Patients with a proximal location to the rectum UC (i.e. not limited to rectal inflammation). 5. Mayo Endoscopic Scale score  1 point 6. Subjects with current immunizations in accordance with the adopted immunization program in Poland - vaccination with live vaccines. 7. Informed written consent to participate in the study signed by legal representatives and subjects from 13 years of age