BCG in Combination With Durvalumab in Adult BCG-naïve, High-risk NMIBC Participants

Inclusion Criteria: * BCG-naïve (defined as participants not having received prior intravesical BCG or who previously received but stopped BCG more than 3 years before study entry). * Local histological confirmation (based on cytology and/or pathology report) of high-risk transitional cell carcinoma of the urothelium of the urinary bladder confined to the mucosa or submucosa. * Complete resection of all Ta/T1 papillary disease prior to enrollment, with the transurethral resection of bladder tumor (TURBT) removing high-risk NMIBC (non-muscle invasive bladder cancer) performed not more than 4 months before enrollment in the study. * No prior radiotherapy for bladder cancer. * A life expectancy of at least 12 weeks (90 days). * Adequate organ and marrow function * World Health Organization/Eastern Cooperative Oncology Group performance status of 0 or 1 at screening * No prior exposure to immune-mediated therapy of cancer * A candidate for BCG treatment. * Local histological confirmation (based on cytology and/or pathology report) of high-risk transitional cell carcinoma of the urothelium of the urinary bladder confined to the mucosa or submucosa. A high-risk tumor is defined as one of the following: T1 tumor; High-grade/G3 tumor; CIS. Exclusion Criteria: * Evidence of muscle-invasive, locally advanced, metastatic, and/or extra-vesical bladder cancer (ie, T2, T3, T4, and / or Stage IV). * Predominantly variant histology such as micropapillary, plasmacytoid, nested, sarcomatoid, microcystic, squamous and adeno variants of UC representing \> 50% of tumor tissue or other than urothelial tumors as assessed by pathology. * Evidence of lymphovascular invasion of bladder tumor, except if treatment with BCG is deemed to be the only clinically viable treatment. * Immediate cystectomy is indicated. * Known or documented absolute and/or relative contraindication of adjuvant intravesical BCG treatment. * Concurrent extravesical, non-muscle-invasive transitional cell carcinoma of the urothelium. * History of allogenic organ transplantation. Participants with any history of allogenic stem cell transplantation are also excluded. * Active or prior documented autoimmune or inflammatory disorders. * Participants with hypothyroidism stable on hormone replacement. * History of active primary immunodeficiency. * Active infection including hepatitis B (known positive HBV/HBsAg result), HCV, or HIV 1/2 (positive HIV) antibodies. * Current or prior use of immunosuppressive medication within 14 days before the first durvalumab dose. * Female participants who are pregnant or breastfeeding or male or female participants of reproductive potential who are not willing to employ highly effective birth control. * Any concurrent chemotherapy, study intervention, biologic or hormonal therapy for cancer treatment; uncontrolled intercurrent illness; * History of another primary malignancy except for Malignancy treated with curative intent and with no known active disease ≥ 2 years; Adequately treated nonmelanoma skin cancer or lentigo maligna without evidence of disease; Adequately treated CIS without evidence of disease; Prostate cancer of stage ≤ T2cN0M0 without biochemical recurrence or progression that in the opinion of the Investigator does not require active intervention. * Previous or concurrent treatment with potent systemic immunostimulatory agents