Inclusion Criteria:  Histologically confirmed invasive breast carcinoma Centrally-confirmed human epidermal growth factor receptor 2 (HER2)-positive invasive breast cancer Centrally confirmed PD-L1 and hormone receptor status Clinical stage at disease presentation: cT4/anyN/M0, any cT/N2-3/M0, or cT1-3/N0-1/M0 (participants with cT1mi/T1a/T1b/N0 are not eligible) Completion of pre-operative systemic chemotherapy including at least 9 weeks of taxane and 9 weeks of trastuzumab (anthracycline and/or additional HER2-targeted agents are permitted) <=12 weeks between primary surgery and randomization Eastern Cooperative Oncology Group (ECOG) Performance Status 0 or 1 Screening left ventricular ejection fraction (LVEF) >= 50% and no decrease in LVEF by >15% from the pre-chemotherapy LVEF. If no pre-chemotherapy LVEF, screening LVEF >= 55% Life expectancy >= 6 months Adequate hematologic and end organ function  Exclusion Criteria:  Stage IV breast cancer An overall response of disease progression according to the investigator at the conclusion of preoperative systemic therapy Prior treatment with T-DM1, or atezolizumab, or other immune checkpoint inhibitors History of exposure to various cumulative doses of anthracyclines History of other malignancy within 5 years prior to screening, except for appropriately treated carcinoma in situ of the cervix, non-melanoma skin carcinoma, Stage I uterine cancer, or ductal carcinoma in situ (DCIS) Current grade >=2 peripheral neuropathy History of idiopathic pulmonary fibrosis, organizing pneumonia, or pneumonitis History of or active autoimmune disease or immune deficiency Treatment with immunostimulatory or immunosuppressive agents Cardiopulmonary dysfunction Any known active liver disease