A Phase III, Multisite, Randomized, Double-Blind Trial of BNT327 in Combination With Chemotherapy Versus Placebo With Chemotherapy in Patients With Previously Untreated Locally Recurrent Inoperable or Metastatic TNBC Determined Ineligible for PD(L)1 Therapy Based on PD-L1 Negative Disease

Inclusion Criteria: * Are considered ineligible for combination treatment with a monospecific PD(L)1 targeting immunotherapy plus chemotherapy as per their tumor PD-L1 expression status. * Have confirmed locally recurrent inoperable or metastatic TNBC, or estrogen receptor (ER)-low, human epidermal growth factor receptor 2 (HER2)-negative breast cancer (ER and/or progesterone receptor \[PgR\]) 1% to 10%, HER2 immunohistochemistry \[IHC\] 0, 1+, or 2+ with fluorescence in situ hybridization \[FISH\] negative for HER2 gene amplification) documented prior to trial screening as part of standard of care. * Have at least one measurable lesion as the targeted lesion based on RECIST v1.1. * Have provided a tissue sample, archival or fresh, during the screening period (bone biopsies, fine needle aspiration biopsies, and samples from pleural or peritoneal fluid are not acceptable; participants with only one target lesion are not eligible to participate in the trial). * Eastern cooperative oncology group (ECOG) performance status of 0 or 1. Exclusion Criteria: * Have received any of the following therapies or drugs prior to the initiation of trial: * Have received prior systemic anticancer therapy for advanced disease. * Have received prior treatment with a PD(L)-1/vascular endothelial growth factor (VEGF) bispecific antibody. * Have received systemic corticosteroids (at a dosage greater than 10 milligrams \[mg\]/day of prednisone or an equivalent dose of other corticosteroids) within 7 days prior to the initiation of trial treatment. Exception: excluding local, intranasal, intraocular, intra-articular or inhaled corticosteroids, short-term use (\<= 7 days) of corticosteroids for prophylaxis (for example, prevention of contrast agent allergy) or treatment of non-autoimmune conditions (for example, delayed hypersensitivity reactions caused by exposure to allergens). * Have been vaccinated with live attenuated vaccine(s) within 4 weeks prior to initiation of trial treatment. * Have received broad-spectrum intravenous antibiotics therapy within 2 weeks prior to initiation of trial treatment. * Are pregnant or breastfeeding or are planning pregnancy or planning to father children during the trial or within 6 months after the last dose of pumitamig or placebo. * Have undergone major organ surgery, significant trauma, or invasive dental procedures (such as dental implants) within 28 days prior to the initiation of trial treatment or plan to undergo elective surgery during the trial. Placement of vascular infusion devices is allowed. * Have received allogeneic hematopoietic stem cell transplantation or organ transplantation.