This is a multicenter, double-blind, randomized study to compare the safety, effectiveness and immunogenicity (antibodies against drug) of two biological drugs administered intravenously - vedolizumab (a drug not registered in pediatric patients) and infliximab (used as standard therapy), in pediatric patients with UC aged 6-18 years.

Before enrollment to the study, informed consent must be obtained from the participant's legal guardian and, additionally, from the participant aged >16 years before any procedures are performed.

Inclusion criteria

1. Participant aged 6 to 17 years at the time of study entry.

2. Patient with moderate or severe UC defined by the PUCAI score (min. 30 points) - diagnosed at least 1 month before the qualifying visit.

3. Individuals who failed to respond to, lost response to, or were intolerant to treatment with at least 1 of the following agents:

   • corticosteroids (prednisone at a dose of 1 mg/kg/day with a maximum dose of 40 mg/day, budesonide MMX 9 mg/day) or immunomodulating drugs (e.g. AZA, 6-MP, MTX).
   • in the case of 5 - ASA adrugs, corticosteroids, immunomodulating drugs, doses should remain stable for 2 weeks before administration of the study drug

4. Patients with UC proximal to the rectum (i.e. not limited to proctitis).

5. At least Mayo 1 on the endoscopic scale

6. People with up-to-date vaccinations in accordance with the preventive vaccination program adopted in Poland.

Exclusion criteria

1. People who have been previously treated with biological antiintegrin preparations, including: natalizumab, efalizumab, etrolizumab or addressin-1 adhesion molecule antagonists or rituximab.

2. People who were previously treated with vedolizumab.

3. People who were previously treated with infliximab

4. People with hypersensitivity or allergy to any of the excipients of vedolizumab or infliximab

5. People with active brain/meningeal disease, signs/symptoms or history of progressive multifocal leukoencephalopathy (PML) or any other serious neurological disorder including stroke, multiple sclerosis, brain tumor or neurodegenerative disease.

6. Individuals who currently require or are expected to require surgical intervention for UC during this study.

7. People who have had a partial or total colectomy or have a jejunostomy, ileostomy, colostomy, ileo-anal stoma, or known permanent intestinal stricture.

8. People with a current diagnosis of unspecified colitis.

9. People with clinical features suggestive of monogenic inflammatory bowel disease of very early onset.

10. The participant has other serious comorbidities that will limit his or her ability to complete the study.

11. Abnormal laboratory tests:

    • Hgb ≤ 8 g/dl
    • Leukocytes ≤ 2.5 x 10*9 cells/L
    • ALT and/or AST ≥ 3 x upper limit of normal
    • positive Quantiferon result
    • positive result in the stool sample for Clostridium difficile and/or stool culture for alarm pathogens Security

At every check-up visit:

• occurrence of: adverse events (AEs), serious AEs (serious AEs-SAEs), infections including opportunistic infections such as PML, liver damage, malignant tumors, infusion-related reactions and hypersensitivity
• vital signs, results of standard laboratory tests

Allocation to therapeutic subgroups will be in a 1:1 ratio.