Abstract Background: Immune-based therapy for metastatic breast cancer has had limited success. Strategies to augment adaptive immunity include vaccines targeting genomic amplifications like Human Epidermal Growth Factor Type 2 (HER2), an established driver of malignancy. Using a novel alphaviral vector, we constructed a vaccine encoding a portion of HER2 (VRP-HER2). Methods: In preclinical studies, mice were immunized before or after implantation of hHER2+ tumor cells and HER2-specific immune responses and anti-tumor function were assessed. We then translated this vaccine into a phase I clinical trial in which subjects with advanced HER2-overexpressing breast cancers received VRP-HER2 every 2 weeks for a total of three doses (cohort 1). In cohort 2, subjects received the same dose of VRP-HER2 along with a standard HER2 targeted therapy. Results: VRP-HER2 induced HER2-specific T cell and antibody responses while controlling tumor growth in murine models. Vaccination with VRP-HER2 was well tolerated in both patient cohorts. PFS was modest, while median OS was 50.2 months in cohort 1 and 32.7 months in cohort 2. In cohort 2, there is one partial response and two patients with continued stable disease. Vaccine induced anti-HER2 antibodies and T cells were identified. Increased perforin expression by memory CD8 T cells post vaccination significantly correlated with improved PFS. Conclusions: VRP-HER2 led to an increase in perforin expressing HER2-specific memory CD8 T cells in preclinical and clinical studies, and had profound antitumor effects in murine models. The generation of HER2-specific memory CD8 T cells was significantly correlated with increased PFS in patients. Subsequent studies will seek to enhance T cell activity by combination with anti-PD-1/PD-L1 antibodies. Citation Format: Crosby EJ, Gwin WR, Chang S, Maecker HT, Lubkov V, Snyder JC, Broadwater G, Hyslop T, Osada T, Hobeika AC, Hartman ZC, Morse MA, Lyerly HK. CD8 T cells induced by novel alphaviral vector predict improved progression free survival in advanced HER2+ breast cancer patients [abstract]. In: Proceedings of the 2018 San Antonio Breast Cancer Symposium; 2018 Dec 4-8; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2019;79(4 Suppl):Abstract nr P2-09-16.