Randomization between the two treatment groups will occur in a 1:1 allocation ratio using an institutional balancing algorithm. Patients will be blinded as to whether they are receiving trastuzumab + nelipepimut-S/GM-CSF or trastuzumab + GM-CSF alone. 4.3 Study treatment 24 Protocol: 2014-0443 Version 2.1 Version Date: 11 April 2016 4.3.1 Trastuzumab All patients will receive trastuzumab as per current standard practice. Trastuzumab will be administered as per institutional protocol. 4.3.2 Vaccine 4.3.2.1 Dosage and preparation The E75 vaccine (nelipepimut-S; as 1.5 mg/mL E75 acetate solution) and GM- CSF (as 250 mcg lyophilized Leukine) will be supplied by Galena Biopharma. Galena Biopharma also provides DTH skin test kits, each of which contains 1.5 mg/mL E75 acetate solution. GM-CSF is a potent cytokine that stimulates granulocytes (neutrophils, eosinophils and basophils) and monocytes, but not lymphocytes, erythrocytes, or platelets. Recombinant human GM-CSF is a 14.6 kDa globular protein consisting of 128 amino acids containing 2 intramolecular disulfide bonds and 2 potential N- linked glycosylation sites. Known adverse effects of GM-CSF include bone pain, allergic reactions, lethargy, malaise, anorexia, skin rashes, flushing, fever, and chills. At doses higher than planned in this study, weight gain may be seen along with breathing difficulties, blood clots, and collections of fluid around the heart or lungs, dyspnea, thromboembolic phenomena, and pleural and pericardial effusion. Leukine® (sargramostim) is a recombinant human granulocyte-macrophage colony stimulating factor (rhu GM-CSF) produced by recombinant DNA technology in a yeast (S. cerevisiae) expression system.