The goal of the proposed research is to complete a clinical trial that evaluates the ability of the combination of trastuzumab and the HER2-derived vaccine NeuVaxTM (nelipepimut-S administered with the immunoadjuvant, GM-CSF) in the adjuvant setting to prevent metastatic disease in high-risk HER2-positive breast cancer patients. We hypothesize that combination therapy with trastuzumab plus vaccination is a therapeutic modality that has minimal toxicity and will prevent disease recurrence. Over the course of the funding period, we completed accrual to the trial outlined in Specific Aim 1. Primary vaccination series were completed for all randomized patients and booster inoculations were completed for all but 3 patients (as of 12/2020). An interim analysis has also been completed and overall the intervention was well tolerated. Across 2400 follow up visits, 16 instances (0.7%) of serious adverse events (AEs) were reported. The majority (75.8%) of the AEs were anticipated and the AEs were about equally divided between local and systemic toxicities. Dose reduction was required for 36 doses (1.5%) administered. Twelve recurrences and 1 death has been reported during the follow up period for an overall estimated 3 year DFS of 82%. Patients enrolled on the trial had the immune response assessed in vivo using a delayed type hypersensitivity (DTH) reaction and in vitro by assessing for the presence of E75-specific cytotoxic lymphocytes (CTL) in their peripheral blood. The DTH data has been acquired and is being analyzed; blood has been collected at all designated timepoints and dextramer assays are being performed to assess the E75-CTL response. Additional blood has been collected to establish a biobank for completion of future immune correlative studies (aim #3). Blood samples for immunologic monitoring are being collected at the specified time points, processed, and stored for the planned analyses described in specific aims 2 and 3.