Patient description: A 17 year old boy complains of vomiting, non-bloody diarrhea, abdominal pain, fever, chills and loss of appetite for the past 3 days. He ate a salad at a restaurant prior to his diarrhea onset. Physical exam was remarkable for pallor, jaundice, and diffuse abdominal tenderness. Lab results were as follows: Hemoglobin: 9.7 g/dL Platelet: 110,000 /cu.mm Creatinine: 3.6 mg/dL blood urea nitrogen (BUN): 73 mg/dL direct bilirubin: 2.4 mg/dL lactate dehydrogenase (LDH): 881 IU/L (normal: 110-265 IU/L) Peripheral blood smear showed a moderate number of schistocytes and helmet cells. Shiga-like toxin-producing E. coli (STEC) stx1/stx2 were found in stools. He has no other underlying disease and he is not on any medications.

Clinical trial eligibility criteria: eligible ages (years): 2.0-17.0,  Ulcerative Colitis Evidence of a personally signed and dated informed consent document and assent document Males and females 2 to less than18 years old and weighing at least 10 kg Having a pathology report that confirms colonic inflammation consistent with UC with a clinical diagnosis of UC for at least 12 weeks prior to baseline, with biopsy report supporting the diagnosis of UC Participants diagnosed with UC at age less than 6 years old, must have had testing and be negative for monogenic disorders associated with very early onset IBD Moderately to severely active UC as defined (via screening colonoscopy) by a Mayo score of at least 6, with a rectal bleeding score of at least 1 and an endoscopic subscore of at least 2 Pediatric Ulcerative Colitis Activity Index (PUCAI) score greater or equal to 35  No history of dysplasia or colon cancer No evidence or history of untreated or inadequately treated active or latent infection with Mycobacterium Tuberculosis For participants outside of the United States: have had an inadequate response or been intolerant to at least one prior therapy as listed below or have a medical contraindication to such therapies Oral or intravenous (IV) corticosteroids  Diagnosis of indeterminate colitis, isolated proctitis, microscopic colitis, infectious colitis, Crohn's disease, or clinical findings suggestive of Crohn's disease History of symptomatic obstructive intestinal strictures or active ostomy, or history of colectomy, extensive small bowel resection ( greater than100 centimetres) or short bowel syndrome, or hospitalization for UC related reason(s) within 4 weeks of baseline visit Any factors or clinical characteristics potentially related to the risk of venous thromboembolism that may increase the risk associated with study participation or study intervention administration or may interfere with the interpretation of study results and, in the judgment of the investigator, would make the participant inappropriate for entry into this study Participants who have previously received tofacitinib or another Janus Kinase inhibitor Vaccination or exposure to a live or attenuated vaccine within the 6 weeks prior to the first dose of study drug, or who are expected to be vaccinated or to have household exposure to these vaccines during treatment or during the 6 weeks following discontinuation of study drug Participants having received azathioprine, 6-mercaptopurine, methotrexate, thioguanine, infliximab, adalimumab, golimumab, interferon, cyclosporine, mycophenolate, tacrolimus, IV or rectally administered corticosteroids, natalizumab, vedolizumab, other antiadhesion molecules, or investigational drugs during the specified time periods prior to baseline whereby they may still have pharmacokinetic and/or pharmacodynamic effect in the body of the participant Previous treatment by leukocyte apheresis including selective lymphocyte, monocyte, or granulocyte apheresis, or plasma exchange within 6 months prior to baseline Treatment by specified prohibited concomitant medications, including moderate to potent CYP3A inducers or inhibitors in the specified time periods prior to the first dose of study drug or are expected to receive any of these medications during the study period Chronic and frequent use of antimotility agents for control of diarrhea (ie, diphenoxylate hydrochloride with atropine sulfate or loperamide) History of bowel surgery, including cholecystectomy within 6 months prior to baseline, history of appendectomy within 3 months prior to baseline, or significant trauma or major surgery within 4 weeks of screening visit are excluded