Statement: Only patients with HER2+ve breast cancer are eligible for the primary trial
Primary_evidence: Inclusion Criteria:   Archival tumor samples must be obtained from primary and/or metastatic sites   Able to submit tumor tissue that is evaluable for programmed death- ligand 1 (PD-L1) expression   HER-2 positive BC as defined by an immunohistochemistry score of 3 or gene amplified by in-situ hybridization as defined by a ratio of greater than or equal to (>=) 2.0 for the number of HER2 gene copies to the number of chromosome 17 copies   Histologically or cytologically confirmed invasive BC: incurable, unresectable, locally advanced BC previously treated with multimodality therapy or metastatic BC   Prior treatment for BC in the: adjuvant; unresectable locally advanced; or metastatic settings; which must include both, a taxane and trastuzumab (alone or in combination with another agent)   Progression must have occurred during or after most recent treatment for locally advanced/metastatic BC or within 6 months after completing adjuvant therapy   Participants must have measurable disease that is evaluable as per RECIST v1.1   Eastern Cooperative Oncology Group Performance Status of 0 or 1   Negative serum pregnancy test within 7 days of enrollment for pre-menopausal women and for women less than 12 months after the onset of menopause   Use of highly effective method of contraception as defined by the protocol Exclusion Criteria:   Prior treatment with trastuzumab emtansine, cluster of differentiation 137 agonists, anti-programmed death-1, or anti-PD-L1 therapeutic antibody or pathway-targeting agents   Receipt of any anti-cancer drug/biologic or investigational treatment within 21 days prior to Cycle 1 Day 1 except hormone therapy, which can be given up to 7 days prior to Cycle 1 Day 1; recovery of treatment related toxicity consistent with other eligibility criteria   Radiation therapy within 2 weeks prior to Cycle 1, Day 1   History of exposure to the cumulative doses of anthracyclines   History of other malignancy within the previous 5 years, except for appropriately treated carcinoma in situ of the cervix, non-melanoma skin carcinoma, Stage I uterine cancer, or participants who have undergone potentially curative therapy with no evidence of disease and are deemed by the treating physician to be at low risk for recurrence   Cardiopulmonary dysfunction, symptomatic pleural effusion, pericardial effusion, or ascites   Participants with severe infection within 4 weeks prior to randomization, including but not limited to hospitalization for complications of infection, bacteremia, or severe pneumonia   Current severe, uncontrolled systemic disease   Major surgical procedure or significant traumatic injury within 28 days prior to randomization or anticipation of the need for major surgery during the course of study treatment   Clinically significant history of liver disease, including cirrhosis, current alcohol abuse, autoimmune hepatic disorders, sclerosis cholangitis or active infection with human immunodeficiency virus, hepatitis B virus, or hepatitis C virus   Need for current chronic corticosteroid therapy (>=10 mg of prednisone per day or an equivalent dose of other anti-inflammatory corticosteroids)   Spinal cord compression not definitively treated with surgery and/or radiation, or previously diagnosed and treated spinal cord compression without evidence that disease has been clinically stable for greater than (>) 2 weeks prior to randomization   Participants with known central nervous system disease   Leptomeningeal disease   History of autoimmune disease   Prior allogeneic stem cell or solid organ transplantation   Active tuberculosis   Receipt of a live, attenuated vaccine within 4 weeks prior to randomization or anticipation that such a live, attenuated vaccine will be required during the study   Treatment with systemic immunostimulatory agents within 4 weeks or five half-lives of the drug (whichever is shorter) prior to randomization   Treatment with systemic corticosteroids or other systemic immunosuppressive medications within 2 weeks prior to randomization, or anticipated requirement for systemic immunosuppressive medications during the trial   Participants who are breastfeeding, or intending to become pregnant during the study