Inclusion Criteria Stratum A (Adjuvant patients) & B (Post-neoadjuvant patients)

          1. Male or female subjects aged > 18 years

          2. Signed written informed consent before any trial-related procedure is undertaken that
             is not part of the standard patient management

          3. Eastern Cooperative Oncology Group (ECOG) performance status 0 or 1

          4. Patients must have completed treatment with curative intent including: surgery and
             adjuvant chemotherapy.

          5. Patients must have completed adjuvant chemotherapy including at least 3 courses of an
             anthracycline agent and 3 courses of a taxane agent. Patients who received dose-dense
             regimens and those who received carboplatin as part of the adjuvant treatment are
             eligible.

          6. No more than 10 weeks may elapse between the completion of last adjuvant treatment
             (adjuvant chemotherapy or surgery) and randomization.

        8. Normal organ and marrow function

          1. White blood count (WBC) greater than or equal to 2.5 x109/L

          2. Absolute neutrophil count (ANC) greater than or equal to 1.5 x109/L

          3. Absolute lymphocyte count greater or equal to 0.5 x109/L

          4. Platelet count greater than or equal to 100 x109/L

          5. Hemoglobin greater than or equal to 9 g/dL

          6. Serum creatinine less or equal to 1.5 x the upper limit of laboratory normal range
             (ULN)

          7. Adequate hepatic function defined by a total bilirubin level less or equal to 1.5 x
             ULN range and AST and ALT levels less or equal than 2.5 x ULN for all subjects. For
             patients with known Gilbert's syndrome, total bilirubin levels less or equal than 2 x
             ULN range (with direct bilirubin less than ULN) will be accepted.

             9. Highly effective contraception (i.e. methods with a failure rate of less than 1 %
             per year) for both male and female subjects if the risk of conception exists (Note:
             The effects of the trial treatment on the developing human fetus are unknown; thus,
             women of childbearing potential and men must agree to use highly effective
             contraception, defined in Appendix A or as stipulated in national or local guidelines.
             Highly effective contraception must be used 28 days prior to first trial treatment
             administration, for the duration of trial treatment, and at least for 60 days after
             stopping trial treatment. Should a woman become pregnant or suspect she is pregnant
             while she or her partner is participating in this trial, the treating physician should
             be informed immediately).

             10. Ability to understand and willingness to sign a written informed consent.

        Inclusion Criteria Stratum A (Adjuvant patients)

          1. Non-metastatic, histologically confirmed primary invasive breast carcinoma

          2. Triple negative breast cancer: hormone receptor negative (ER < 10% and PgR < 10%) and
             HER2 negative (IHC 0/1+ or ISH non-amplified), as defined by the local pathology
             laboratory. In case of discordance between pre-operative core-biopsy and the surgical
             sample, the receptor assessment performed on the surgical sample has to be considered
             for inclusion criteria evaluation.

          3. Availability of a formalin-fixed, paraffin-embedded block containing tumor tissue or
             at least 7 unstained tumor slides.

          4. Adequately excised: patients must have undergone either breast-conserving surgery or
             mastectomy/nipple- or skin-sparing mastectomy. The margins of the resected specimen
             should be free of invasive tumor and ductal carcinoma in situ (no ink on tumor). In
             the case of breast-conserving surgery patients with margins positive for lobular
             carcinoma in situ (LCIS) are eligible without additional resection. For patients who
             undergo mastectomy, patients with a microscopic positive deep margin are eligible,
             provided they will receive radiotherapy on chest wall.

          5. Patients must have had axillary lymph node dissection for evaluation of pathologic
             nodal status. Only patients in one of the following stage categories will be eligible:

               -  if 4 or more metastatic lymph nodes, any pT

               -  if 1 to 3 metastatic lymph nodes, pT >2 cm

               -  if no metastatic lymph nodes, pT >5 cm

        Inclusion criteria:

        Stratum B (Post-neoadjuvant patients)

          1. Non-metastatic histologically confirmed invasive breast carcinoma.

          2. Triple negative breast cancer: hormone receptor negative (ER < 10% and PgR < 10%) and
             HER2 negative (IHC 0/1+ or ISH non-amplified), as defined by the local pathology
             laboratory. In case of discordance between the pre-treatment diagnostic core-biopsy
             and the surgical sample, the receptor assessment performed on the surgical sample has
             to be considered for inclusion criteria evaluation.

          3. Adequately excised: patients should have undergone adequate tumor excision after
             preoperative chemotherapy, which means surgical removal of all clinically evident
             disease in the breast and lymph nodes.

               1. Breast surgery: patients must have undergone either breast-conserving surgery or
                  mastectomy/nipple- or skin-sparing mastectomy. The margins of the resected
                  specimen should be free of invasive tumor and ductal carcinoma in situ (no ink on
                  tumor). In the case of breast-conserving surgery patients with margins positive
                  for lobular carcinoma in situ (LCIS) are eligible without additional resection.
                  For patients who undergo mastectomy, patients with a microscopic positive deep
                  margin are eligible, provided they will receive radiotherapy on chest wall.

               2. Lymph node surgery:

             i. Axillary dissection without sentinel node evaluation is permitted after
             preoperative therapy.

             ii. In case of positive results from a fine-needle aspiration, core biopsy, or
             sentinel node biopsy performed prior to preoperative therapy, additional surgical
             evaluation of the axilla following preoperative therapy is required.

             iii. If sentinel node biopsy performed before preoperative therapy was negative, no
             additional surgical evaluation of the axilla is required after preoperative therapy.

             iv. Sentinel node after preoperative therapy is allowed if no evidence of axillary
             node involvement was documented by ultrasonography at diagnosis. If sentinel node
             biopsy after preoperative therapy is negative, no further additional surgical
             evaluation of the axilla is required. If sentinel node biopsy performed after
             preoperative therapy is positive, additional surgical evaluation of the axilla is
             recommended.

          4. Pathologic evidence of residual invasive carcinoma in the breast and/or axillary lymph
             nodes on the surgical specimen obtained after preoperative therapy (ypT1micN0,
             ypT1micN0i+, ypT0N0i+ will be excluded).

          5. Clinical stage at presentation: T1-4, N0-3, M0 (Exception: Patients with T1a/bN0
             tumors at presentation will not be eligible).

          6. No more than 10 weeks may elapse between the date of last treatment (surgery or
             post-surgery chemotherapy if indicated) and the date of randomization. In case of
             positive margins after the first intervention requiring additional resection.

          7. Availability of a formalin-fixed, paraffin-embedded block containing tumor tissue or
             at least 7 unstained tumor slides (tumor sample from the diagnostic core-biopsy
             obtained before neoadjuvant chemotherapy). In case only 7 unstained slides from the
             bioptic sample will be available, the investigator must ensure that the sample
             contains tumor tissue by performing an hematoxylin and eosin staining.

        Exclusion criteria: Stratum A (Adjuvant patients) & B (Post-neoadjuvant patients)

          1. Stage IV breast cancer.

          2. History of any prior (ipsi- and/or contralateral) invasive breast carcinoma diagnosed
             within 10 years.

          3. Synchronous bilateral breast cancer, unless both tumors confirmed as triple negative
             disease.

          4. History of non-breast malignancies within the 5 years prior to study entry, except for
             the following: Carcinoma in situ (CIS) of the cervix, CIS of the colon, Basal cell and
             squamous cell carcinomas of the skin.

          5. Prior organ transplantation, including allogeneic stem-cell transplantation.

          6. Prior or concomitant treatment with any other investigational agents.

          7. Prior therapy with any antibody / drug targeting T-cell coregulatory proteins
             (immune-checkpoints) such as PD-1, PD-L1, or cytotoxic T-lymphocyte antigen-4
             (CTLA-4).

          8. Concurrent anticancer treatment (for example, cytoreductive therapy, immune therapy,
             or cytokine therapy except for erythropoietin)

          9. Major surgery for any reason, within 4 weeks of randomization and / or if the subject
             has not fully recovered from the surgery within 4 weeks of randomization.

         10. Concomitant treatment with all herbal (alternative) remedies with immunostimulating
             properties (for example, mistletoe extract) or known to potentially interfere with
             major organ function (for example, hypericin).

         11. Subjects receiving immunosuppressive agents (such as steroids) for any reason should
             be tapered off these drugs before initiation of the trial treatment (with the
             exception of subjects with adrenal insufficiency, who may continue corticosteroids at
             physiologic replacement dose, equivalent to ≤ 10 mg prednisone daily).

         12. Significant acute or chronic infections including, among others:

               1. Known history of testing positive test for human immunodeficiency virus (HIV) or
                  known acquired immunodeficiency syndrome.

               2. Hepatitis B virus (HBV) or hepatitis C virus (HCV) infection at screening
                  (positive HBV surface antigen or HCV RNA if anti-HCV antibody screening test
                  positive).

         13. Active autoimmune disease that might deteriorate when receiving an immunostimulatory
             agent:

               1. Subjects with diabetes type I, vitiligo, psoriasis, hypo- or hyperthyroid disease
                  not requiring immunosuppressive treatment are eligible.

               2. Subjects requiring hormone replacement with corticosteroids are eligible if the
                  steroids are administered only for the purpose of hormonal replacement and at
                  doses ≤ 10 mg or equivalent prednisone per day.

               3. Administration of steroids through a route known to result in a minimal systemic
                  exposure (topical, intranasal, intro-ocular, or inhalation) are acceptable.

         14. Administration of steroids through a route known to result in a minimal systemic
             exposure (topical, intranasal, intro-ocular, or inhalation) are acceptable.

         15. Previous or ongoing administration of systemic steroids for the management of an acute
             allergic phenomenon is acceptable as long as it is anticipated that the administration
             of steroids will be completed in 14 days, or that the daily dose after 14 days will be
             ≤ 10 mg per day of equivalent prednisone.

         16. Known severe hypersensitivity reactions to monoclonal antibodies (Grade ≥ 3 NCI-CTCAE
             v 4.03), any history of anaphylaxis, or uncontrolled asthma (that is, 3 or more
             features of partially controlled asthma).

         17. Clinically significant (that is, active) cardiovascular disease: cerebral vascular
             accident /stroke (< 6 months prior to enrollment), myocardial infarction (< 6 months
             prior to enrollment), unstable angina, congestive heart failure (New York Heart
             Association Classification Class ≥ II), or serious uncontrolled cardiac arrhythmia
             requiring medication.

         18. All other significant diseases (for example, inflammatory bowel disease), which, in
             the opinion of the Investigator, might impair the subject's tolerance of trial
             treatment.

         19. Any psychiatric condition that would prohibit the understanding or rendering of
             informed consent.

         20. Vaccination within 4 weeks of the first dose of avelumab and while on trial is
             prohibited except for administration of inactivated vaccines (for example, inactivated
             influenza vaccines).

         21. Known alcohol or drug abuse.

         22. Persisting toxicity related to prior therapy of Grade > 1 NCI-CTCAE v 4.03 (except for
             grade 2 radiodermatitis and grade 2 neuropathy).

         23. Current pregnancy and/or lactation. Refusal to adopt adequate contraception methods.

        Stratum B (Postneoadjuvant patients)

        1. No invasive residual disease in the breast and axilla at pathological examination after
        neoadjuvant chemotherapy. ypT1micN0, ypT1micN0i+, ypT0N0i+ will also be excluded.