• Patient must have signed a written informed consent prior to any trial-related procedures. When the patient is physically unable to give his written consent, a trusted person of his choice, independent from the investigator or the sponsor, can confirm in writing the patient's consent;

• Age ≥ 18 years;

• Eastern Cooperative Oncology Group (ECOG) performance status of 0-2;

• Histologically documented stage II-III breast cancer according to the primary tumour-regional lymph node anatomic staging criteria of the American Joint Committee on Cancer (AJCC), 8th edition as determined by the investigator during radiologic assessment, clinical assessment or both;

• Estrogen receptor (ER) and Progesterone receptor (PR) ≤10%; HER2-negative as per ASCO/CAP guidelines Note: In case of bilateral breast cancer, participation in the study is permitted as long as both tumours are triple negative;

• Patients previously treated with neoadjuvant chemotherapy in combination with pembrolizumab for a minimum of 6 cycles (All systemic chemotherapy must have been completed preoperatively);

• Absence of residual invasive disease in the breast or lymph nodes after the completion of neoadjuvant therapy (Residual ductal carcinoma in situ \[DCIS\] is allowed);

• Have had an adequately excised breast cancer (surgical removal of all clinically evident disease in the breast and lymph nodes) :

∙ Breast surgery: patients must have undergone either breast-conserving surgery or total mastectomy with histologically negative margins for invasive tumour and DCIS. Patients with margins positive for lobular carcinoma in situ (LCIS) are eligible without additional resection.

‣ Lymph node surgery: patients must have had sentinel lymph node biopsy (SLNB) and/or axillary lymph node dissection (ALND) to evaluate the pathologic nodal status;

• Patients that have received adequate locoregional radiation therapy or with planned adequate locoregional radiation therapy;

⁃ Adequate organ and bone marrow functions. All screening lab tests should be performed within 28 days before randomisation;

• Absolute Neutrophil Count (ANC) ≥ 1,000 /µL

∙ Platelets ≥ 100,000 /µL

∙ Hemoglobin ≥ 9 g/dL

∙ Creatinine clearance ≥ 30 mL/min for subject with creatinine levels \> 1.5 x institutional upper limit of normal (ULN)

∙ Total bilirubin ≤ 1.5 x ULN or direct bilirubin ≤ ULN for subjects with total bilirubin levels \> 1.5 ULN (Patients with Gilbert's disease with a total bilirubin ≤ 2.5 x ULN and direct bilirubin within normal limits are permitted)

∙ Aspartate aminotransferase (ASAT) and alanine aminotransferase (ALAT) ≤ 2.5 x ULN

⁃ Randomisation must take place no more than 12 weeks after breast surgery. Adjuvant radiotherapy is authorized. If given, as per investigator discretion it can be given concurrently with pembrolizumab;

⁃ Patients must not be pregnant or nursing (for women of childbearing potential only, a negative serum pregnancy test must be obtained within 7 days of Cycle 1 Day 1);

⁃ Women of childbearing potential and male patients must agree to use 1 effective form of contraception and up to 4 months after the last dose of study drugs;

⁃ Patients should be able and willing to comply with study visits and procedures as per protocol;

⁃ Patients must be affiliated to a Social Security System (or equivalent).