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Background The ExPEC4V vaccine contains 4 Escherichia coli O-antigens (O1A, O2, O6A, O25B) conjugated to exotoxin protein A and is being studied for prevention of Invasive Extraintestinal pathogenic E. coli (ExPEC) Disease (IED). This phase-2 double-blind study assessed safety and immunogenicity of ExPEC4V Clinical Trial Material (CTM), manufactured via a redesigned process (optimized O1A strain). Methods Participants (≥18 years) in stable health were randomized (3:1) to receive ExPEC4V dose 4:4:4:8 μg PS/serotype or placebo on Day 1 and second vaccination on Day 181 (6 months after first vaccination). Participants will be followed for safety until end of study at Day 360. Reactogenicity and immunogenicity (by ELISA, opsonophagocytic killing [OPA] assays) were evaluated pre-vaccination, and 15 days after first and second vaccinations (Day 195). Results Of 100 participants randomized (mean age 56, 48% males) and vaccinated (ExPEC4V, n = 75; placebo, n = 25), 97 completed Day 30. Solicited local AEs were higher for ExPEC4V (38.7%) than placebo (20%); most frequent was pain/tenderness (38.7% vs 20%). Solicited systemic AEs were higher in ExPEC4V (49.3%) than placebo (20%); most frequent was fatigue (32% vs. 12%). No serious or grade 3 solicited local AEs were reported. One participant in ExPEC4V experienced a grade 3 solicited systemic fatigue considered vaccine-related by investigator. ExPEC4V demonstrated immune responses against all serotypes at Day 15. Geometric mean titer effective concentration rank by serotypes was O2 &gt; O1A &gt; O6 &gt; O25B (Figures 1 and 2). At Day 15, ≥ 82% of participants in ExPEC4V and none in placebo had ≥2-fold increase from baseline of ELISA titer for all serotypes. In ExPEC4V, ≥47% had ≥2-fold increase from baseline of OPA titer for all serotypes, while 8% in placebo had ≥2-fold increase only for O6A. Good correlation was observed between ELISA and OPA across serotypes (r ≥ 0.76). Conclusion ExPEC4V elicited robust and functional immune responses across all serotypes and was well tolerated with no vaccine safety findings. This study supports the development of future multivalent ExPEC vaccine to prevent IED. Disclosures All authors: No reported disclosures.

open forum infectious diseases

2712. Safety and Immunogenicity of two Doses of ExPEC4V Vaccine Against Extraintestinal Pathogenic Escherichia coli Disease in Healthy Adult Participants