The Efficacy and Safety of Combining Probiotic VSL#3 With Vedolizumab for the Treatment of Moderate Ulcerative Colitis

Inclusion Criteria: * Subject must be a man or woman aged 18-85 years, inclusive. * Diagnosed with ulcerative colitis (UC) at least 90 days prior to baseline, supported by comprehensive colonoscopy findings and histopathological evidence obtained within the past year. Participants must provide full colonoscopy or sigmoidoscopy reports along with pathology results conducted within the last 3 months, as well as blood indicators (within 1 week) at baseline screening. Individuals currently experiencing infection, dysplasia, or malignancy will be excluded from participation. * Subject has moderate active UC and had a Mayo score of 6-10 at screening. * Planned treatment with Vedolizumab as initial therapy or reinduction, with reinduction defined as no vedolizumab treatment within 1 year. * Confirmed by the investigator that despite receiving at least one of the following treatments, the subject has shown an inadequate response, loss of response, or intolerance: 1. Oral aminosalicylates (e.g., mesalamine, sulfasalazine, olsalazine, balsalazide), where the subject exhibits ongoing signs and symptoms of active disease during at least 4 weeks of treatment with mesalamine 2.4 g/day, sulfasalazine 4 g/day, olsalazine 1 g/day, or balsalazide 6.75 g/day. 2. Immunosuppressants: failure to respond after receiving at least 42 consecutive days of treatment with azathioprine, 6-mercaptopurine, or methotrexate (MTX) injections prior to baseline (Week 0), with minimum doses of azathioprine ≥ 0.75 mg/kg/day or 6-MP ≥ 0.5 mg/kg/day (rounded to the nearest tablet dose) or MTX ≥ 15 mg/week (SC/IM), or the highest tolerated dose due to adverse effects such as leukopenia, elevated liver enzymes, or nausea. * No increase in dose of oral 5-ASA and Immunosuppressants could be allowed if it is maintained stable at least 2 weeks before screening. * Subject must be willing and able to adhere to the prohibitions and restrictions specified in this protocol. * Willing and able to complete the required Subject Diary. * Willing and able to meet all study requirements, including attending all assessment visits and phone calls. Exclusion criteria: * Diagnosis of Crohn's disease, undetermined IBD or other colitis. * UC disease limited to the rectum (\<15 cm from the anal verge) * Steroid therapy initiation within 2 weeks before screening visit. * Used antibiotics for intestinal or other infections within 2 weeks of the screening * Use of other probiotics preparations within the last 2 weeks before study entry (screening) * Used rectal 5-ASA within the past week before study entry (screening) * Adjustment of oral 5-ASA and immunosuppressant dosages due to disease progression after colonoscopy screening until enrollment. * Within 1 week prior to screening, the participant has taken nonsteroidal anti-inflammatory drugs (NSAIDs) or anti-diarrheal medications for 5 consecutive days. * Positive Clostridioides difficile detection toxin results within the past month prior to screening. * Pregnancy and breastfeeding women * Other biologics/advanced therapies are used as concomitant therapy and Previous use of other biologics * History of allergy to maltose and/or cornstarch and/or silica * Subjects with severe primary heart, liver, lung, kidney, hematologic, or serious diseases that affect their survival, such as cancer, AIDS, asthma, kidney stones, renal dysfunction; urine protein \>+, microscopic hematuria, ALT \>2N (N is the upper limit of normal), Cr \> normal upper limit, platelet count \<50x10\^9/L, white blood cell count \<3.0x10\^9/L.