BACKGROUND: We have previously published initial efficacy of the indoleamine 2,3-dioxygenase (IDO)/anti-programmed death ligand 1 (PD-L1) vaccine in combination with nivolumab in 30 anti-PD-1 therapy na•ve patients with metastatic melanoma (cohort A). We now report long-term follow-up of patients in cohort A. Further, we report results from cohort B, where the peptide vaccine was added to anti-PD-1 therapy for patients with progressive disease during anti-PD-1 treatment. METHODS: All patients were treated with a therapeutic peptide vaccine in Montanide targeting IDO and PD-L1 combined with nivolumab (NCT03047928). A long-term follow-up of safety, response rates, and survival rates were performed in cohort A including patient subgroup analyses. Safety and clinical responses were analyzed for cohort B. RESULTS: Cohort A: At data cut-off, January 5, 2023, the overall response rate (ORR) was 80%, and 50% of the 30 patients obtained a complete response (CR). The median progression-free survival (mPFS) was 25.5 months (95%_CI 8.8 to 39), and median overall survival (mOS) was not reached (NR) (95%_CI 36.4 to NR). The minimum follow-up time was 29.8 months, and the median follow-up was 45.3 months (IQR 34.8-59.2). A subgroup evaluation further revealed that cohort A patients with unfavorable baseline characteristics, including either PD-L1 negative tumors (n=13), elevated lactate dehydrogenase (LDH) levels (n=11), or M1c (n=17) obtained both favorable response rates and durable responses. The ORR was 61.5%, 79%, and 88% for patients with PD-L1(-) tumors, elevated LDH, and M1c, respectively. The mPFS was 7.1 months for patients with PD-L1(-) tumors, 30.9 months for patients with elevated LDH, and 27.9 months for M1c patients. Cohort B: At data cut-off, the best overall response was stable disease for 2 of the 10 evaluable patients. The mPFS was 2.4 months (95%_CI 1.38 to 2.52), and the mOS was 16.7 months (95%_CI 4.13 to NR). CONCLUSION: This long-term follow-up confirms the promising and durable responses in cohort A. Subgroup analyses of patients with unfavorable baseline characteristics revealed that high response rates and survival rates were also found in patients with either PD-L1 negative tumors, elevated LDH levels, or M1c. No meaningful clinical effect was demonstrated in cohort B patients. TRIAL REGISTRATION NUMBER: NCT03047928.