• HLA-A\*02-positive, unless being enrolled in the third non-HLA-A\*02 arm

• Histologically confirmed diagnosis of HER2/neu positive primary breast cancer for all tumors biopsied (multifocal, multicentric, or synchronous contralateral disease)

• Completion of both neoadjuvant and adjuvant trastuzumab-based standard of care breast cancer therapy

• Stage I, II, or III at presentation with pathologic evidence of residual invasive carcinoma in the breast or axillary lymph nodes (residual disease) at surgery following completion of neoadjuvant therapy -OR- Stage III at presentation with pathologic complete response (pCR) at surgery following completion of neoadjuvant therapy

• The subject can begin study therapy within one year of completion of adjuvant trastuzumab-based therapy and any other standard therapies, but, study therapy can be administered concurrently with endocrine therapy.

• No clinical evidence of residual or persistent breast cancer per treating physician assessment

• ECOG 0-2

• Adequate organ function

• Negative pregnancy test or evidence of post-menopausal status

• If of childbearing potential, willing to use a form of highly effective contraception

• Subject must both reside in and have been treated for their cancer in the country in which the clinical site is located.