Pembrolizumab vs. Observation in People With Triple-negative Breast Cancer Who Had a Pathologic Complete Response After Chemotherapy Plus Pembrolizumab

Inclusion Criteria: * Age \>= 18 years * Eastern Cooperative Oncology Group (ECOG) Performance Status 0-2 * Triple Negative Breast Cancer: * Patients with a history of clinical stage T1cN1-2 or T2-4N0-2 (clinical stage II or III prior to preoperative therapy) breast cancer at time of diagnosis according to the primary tumor-regional lymph node anatomic staging criteria of the American Joint Committee on Cancer (AJCC), 8th edition as determined by the investigator in radiologic assessment, clinical assessment or both * Patients must have no residual invasive disease in the breast or lymph nodes after the completion of neoadjuvant therapy. Residual ductal carcinoma in situ (DCIS) is allowed. Isolated tumor cells are considered node-negative * Estrogen receptor (ER) and progesterone receptor (PR) =\< 10%; HER2-negative by American Society of Clinical Oncology (ASCO)/College of American Pathologists (CAP) guidelines (immunohistochemistry \[IHC\] and fluorescence in situ hybridization \[FISH\]) * If invasive disease was present in both breasts, participation in the study is permitted as long as the eligibility criteria are met for both tumors/breasts * Patients must have received neoadjuvant chemotherapy in combination with pembrolizumab for a minimum of 6 cycles. All systemic chemotherapy must have been completed preoperatively * An interval of no more than 12 weeks between the completion date of the final surgery and the date of randomization \* Note: Adjuvant radia…