Background: Despite recent therapeutic advances in non-small cell lung cancer (NSCLC), an unmet medical need remains for most patients. Pembrolizumab (pembro) is approved in first-line treatment of patients with NSCLC as a single agent when PD-L1 expression is > 50% and in combination with chemotherapy when PD-L1 expression is < 50%. It is believed that a fraction of non-responders to pembro lack an adequate tumor-directed immune response. CV-301 is a poxviral-based vaccine comprising a prime-boost strategy with Modified Vaccinia Ankara (MVA) prime and fowlpox boost. The viral vectors encode 2 tumor associated antigens, CEA and MUC-1, as well as 3 costimulatory molecules (B7.1, ICAM-1 and LFA-3, called TRICOM). Current preclinical and mechanistic evidence suggests that CV-301 can generate a tumor-directed immune response in NSCLC, potentially increasing the clinical benefit associated with pembro. Methods: This open-label, multi-center trial will evaluate the combination of CV301 and pembro. The phase 1 trial, an evaluation of safety of ascending doses of CV301, has been completed. The vaccine was well tolerated with no DLTs. The phase 1b portion is currently enrolling at least 6 patients with NSCLC in the maintenance setting of pembro and will continue pembro in combination with CV301 with a goal to establish safety of the combination. Safety monitoring will occur every two weeks. If safety is established, the randomized Phase 2 portion will enroll 176 subjects, randomized 2:1 to receive maintence pembro in combination with CV301 (2) or alone (1). Maintenance setting is defined as the period following 11 weeks of initial treatment, after which chemotherapy would be complete in the < 50% PD-L1 expression group. To be eligible, patients must have stable disease or objective response at the time of radiographic evaluation (approximately 11-12 weeks after initiation of first line therapy). Preliminary efficacy will be analyzed during the Phase 2 cohort based on the Intent-to-Treat population, comprising overall survival (primary), progression-free survival and overall response rate (secondary). Clinical trial information: NCT02840994