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)-positiveinvasive 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 noteligible)

• Completion of pre-operative systemic chemotherapy including at least 9 weeks oftaxane and 9 weeks of trastuzumab (anthracycline and/or additional HER2-targetedagents 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 LVEFby >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 theconclusion 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 forappropriately 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