A Study Evaluating the Efficacy and Safety of Adjuvant Atezolizumab or Placebo and Trastuzumab Emtansine for Participants With HER2-Positive Breast Cancer at High Risk of Recurrence Following Preoperative Therapy

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 (prior to neoadjuvant therapy): 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