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

Abstract

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.

Keywords: Bioconjugate vaccine; Controlled human challenge study; Shigella; Shigella flexneri 2a; Vaccine.

Fig. 1

Study Design and Enrolment. Volunteers received 2 doses of the Flexyn2a vaccine or placebo 28 days apart followed by challenge with S. flexneri 2a 28 days after the second dose. Immunological assessments were done before and after vaccination and after challenge.

Fig. 2

Consort Diagram Study enrolment and subject disposition. BMI= body mass index.

Fig. 3

The mean of the cumulative weight (Panel a) and number (Panel b) of loose stools after challenge in the Flexyn2a recipients (yellow line) and the placebo recipients (grey line). The table reports the maximum weight and number of loose stools within any 24 h period after challenge. N = number; g = grams; h = hours; GM = Geometric mean; IQR = interquartile range; SD = standard deviation (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.).

Fig. 4

Constitutional and enteric symptoms experienced by volunteers after Shigella flexneri 2a challenge. P= placebo recipients (n = 29); V= vaccine recipients (n = 30). Colour indicates severity. The p-value for each symptom is along the top and reflects the difference in severity utilizing the modified Ridit Score.

Fig. 5

Shigellosis disease severity score in challenged volunteers by Vaccinee or placebo recipient (Panel a), only those that developed shigellosis by vaccinee or placebo recipient (Panel b) and those that didn't develop shigellosis by vaccinee or placebo recipient status (Panel c).

Fig. 6

Shows the anti-S. flexneri 2a LPS serum IgG antibody titres by study day. Vac= Vaccination. C-1 = 1 day prior to challenge. C7= 7 days post-challenge.

Fig. 7

Panel a shows the prechallenge serum IgG to S. flexneri 2a LPS in the vaccine and placebo recipients broken down by whether they met the primary objective of Shigellosis. Panel b, the relationship between pre-challenge anti-LPS serum IgG to Shigella disease severity score is demonstrated for the recipients of the Flexyn2a vaccine. Panel c demonstrates the relationship of the pre-challenge anti-LPS serum IgG to maximum stool weight (blue circles) and number (red triangles) in 24 h (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.).