Human challenge study with a Shigella bioconjugate vaccine: Analyses of clinical efficacy and correlate of protection

Background: Shigellosis is a major cause of moderate to severe diarrhoea and dysentery in children under 5 years of age in low and middle-income countries. The Flexyn2a vaccine conjugates the O-polysaccharide of Shigella flexneri 2a to Pseudomonas aeruginosa exotoxin A. We describe a Phase 2b proof-of-concept challenge study that evaluated safety, immunogenicity, and efficacy of the Flexyn2a vaccine to protect against shigellosis. Methods: In this randomized, double blind, placebo-controlled trial, healthy adults were randomized 1:1 to receive Flexyn2a (10 µg) or placebo intramuscularly, twice, 4 weeks apart, followed by challenge 4 weeks later with 1500 colony forming units (CFUs) of S. flexneri 2a strain 2457T. The primary outcome was vaccine-induced protection. S. flexneri 2a lipopolysaccharide (LPS)-specific immune responses were assessed. Findings: Sixty-seven subjects were enrolled, 34 received vaccine and 33 placebo. The vaccine was well tolerated; the majority of adverse events were mild in nature. Thirty vaccinees and 29 placebo recipients received the S. flexneri 2a challenge. Vaccination resulted in a 30.2% reduction in shigellosis compared with placebo (13/30 vs. 18/29; p = 0.11; 95% CI -15 to 62.6). Vaccine efficacy was more robust against severe disease, reaching 51.7% (p = 0.015, 95% CI 5.3 to 77.9) against moderate/severe diarrhoea or dysentery concurrent with fever or severe enteric symptoms and 72.4% (p = 0.07) against more severe diarrhoea (≥10 lose stools or ≥1000 g loose stools/24 h). Vaccinated subjects were less likely to need early antibiotic intervention following challenge (protective efficacy 51.7%, p = 0.01; 95% CI 9 to 76.8). In those who developed shigellosis, vaccinated subjects had a lower disease severity score (p = 0.002) than placebo-recipients. Additionally, LPS-specific serum IgG responses in Flexyn2a recipients were associated with protection against disease (p = 0.0016) and with a decreased shigellosis disease score (p = 0.002). Interpretation: The Flexyn2a bioconjugate vaccine was immunogenic, well tolerated and protected against severe illness after Shigella challenge and is a promising Shigella vaccine construct. We identified a strong association between anti-S. flexneri 2a serum IgG and a reduction in disease outcomes. (Clinicaltrials.gov, NCT02646371.) Funding: Funding for this study was through a grant from the Wellcome Trust.