Safety and immunogenicity of the oral, inactivated, enterotoxigenic Escherichia coli vaccine ETVAX in Bangladeshi children and infants: a double-blind, randomised, placebo-controlled phase 1/2 trial

Abstract

Background: Enterotoxigenic Escherichia coli causes diarrhoea, leading to substantial mortality and morbidity in children, but no specific vaccine exists. This trial tested an oral, inactivated, enterotoxigenic E coli vaccine (ETVAX), which has been previously shown to be safe and highly immuongenic in Swedish and Bangladeshi adults. We tested the safety and immunogenicity of ETVAX, consisting of four E coli strains overexpressing the most prevalent colonisation factors (CFA/I, CS3, CS5, and CS6) and a toxoid (LCTBA) administered with or without a double-mutant heat-labile enterotoxin (dmLT) as an adjuvant, in Bangladeshi children.

Methods: We did a randomised, double-blind, placebo-controlled, dose-escalation, age-descending, phase 1/2 trial in Dhaka, Bangladesh. Healthy children in one of three age groups (24-59 months, 12-23 months, and 6-11 months) were eligible. Children were randomly assigned with block randomisation to receive either ETVAX, with or without dmLT, or placebo. ETVAX (half [5·5 × 1010 cells], quarter [2·5 × 1010 cells], or eighth [1·25 × 1010 cells] adult dose), with or without dmLT adjuvant (2·5 μg, 5·0 μg, or 10·0 μg), or placebo were administered orally in two doses 2 weeks apart. Investigators and participants were masked to treatment allocation. The primary endpoint was safety and tolerability, assessed in all children who received at least one dose of vaccine. Antibody responses to vaccine antigens, defined as at least a two-times increase in antibody levels between baseline and post-immunisation, were assessed as secondary endpoints. This trial is registered with ClinicalTrials.gov, NCT02531802.

Findings: Between Dec 7, 2015, and Jan 10, 2017, we screened 1500 children across the three age groups, of whom 430 were enrolled and randomly assigned to the different treatment groups (130 aged 24-59 months, 100 aged 12-23 months, and 200 aged 6-11 months). All participants received at least one dose of vaccine. No solicited adverse events occurred that were greater than moderate in severity, and most were mild. The most common solicited event was vomiting (ten [8%] of 130 patients aged 24-59 months, 13 [13%] of 100 aged 12-23 months, and 29 [15%] of 200 aged 6-11 months; mostly of mild severity), which appeared related to dose and age. The addition of dmLT did not modify the safety profile. Three serious adverse events occurred but they were not considered related to the study drug. Mucosal IgA antibody responses in lymphocyte secretions were detected against all primary vaccine antigens (CFA/I, CS3, CS5, CS6, and the LCTBA toxoid) in most participants in the two older age groups, whereas such responses to four of the five antigens were less frequent and of lower magnitude in infants aged 6-11 months than in older children. Faecal secretory IgA immune responses were recorded against all vaccine antigens in infants aged 6-11 months. 78 (56%) of 139 infants aged 6-11 months who were vaccinated developed mucosal responses against at least three of the vaccine antigens versus 14 (29%) of 49 of the infants given placebo. Addition of the adjuvant dmLT enhanced the magnitude, breadth, and kinetics (based on number of responders after the first dose of vaccine) of immune responses in infants.

Interpretation: The encouraging safety and immunogenicity of ETVAX and benefit of dmLT adjuvant in young children support its further assessment for protective efficacy in children in enterotoxigenic E coli-endemic areas.

Funding: PATH (Bill & Melinda Gates Foundation and the UK's Department for International Development), the Swedish Research Council, and The Swedish Foundation for Strategic Research.

Copyright © 2020 The Author(s). Published by Elsevier Ltd. This is an Open Access article under the CC BY 4.0 license. Published by Elsevier Ltd.. All rights reserved.

Figures

Figure 1

Geometric mean times increases in ALS (A) and plasma (B) IgA responses in children aged 12–59 months Horizontal lines indicate the geometric mean response for the group, whereas the circles represent individual responses. Times increases were calculated as the post-immunisation response (the highest response on either day 7 after the first dose or day 5 after the second dose) divided by pre-immunisation antibody level. Geometric mean times increases in ALS concentrations were significantly different between vaccine and placebo recipients for all antigens in both age groups (all pt test and all p<0·0005 with Holm's-Bonferroni adjustment). In children aged 24–59 months, geometric mean times increases in plasma IgA concentrations were significantly different between vaccine (n=75) and placebo (n=50) recipients for all antigens (all p<0·0001 with Student's t test and all p=0·0005 with Holm's-Bonferroni adjustment); in children aged 12–23 months, the difference between vaccine (n=60) and placebo (n=40) recipients was significant only for CFA/I, CS3, and LTB (all p≤0·0061 with Student's t test and all p≤0·018 with Holm's-Bonferroni adjustment). ALS=antibodies in lymphocyte secretions.

Figure 2

Geometric mean times increases in faecal secretory IgA (A) and plasma IgA (B) responses in infants aged 6–11 months Horizontal lines indicate the geometric mean response for the group, whereas the circles represent individual responses. Times increases were calculated as the post-immunisation response (the highest response on either day 7 after the first dose or day 5 or 14 after the second dose) divided by pre-immunisation antibody levels. Faecal secretory IgA responses were the ratio between antigen-specific faecal secretory IgA antibody titres divided by total secretory IgA in preimmunisation and corresponding postimmunisation samples. Significant differences between vaccine (n=131) and placebo (n=46) recipients in faecal secretory IgA times increases were observed for CFA/I (p=0·0050), CS3 (p=0·0005), CS5 (p=0·026), CS6 (p=0·028), and LTB (p=0·0020) before Holm's-Bonferroni adjustment and for CFA/I (p=0·015), CS3 (p=0·0025), and LTB (p=0·0080) after adjustment. Significant differences in geometric mean times increases in plasma IgA between vaccine (n=148) and placebo (n=50) recipients were seen for CS3 (p=0·013) and LTB (p

Figure 3

Frequencies of mucosal IgA (ALS or faecal, or both) and plasma IgA responses in all age groups Frequencies of mucosal (A) and plasma (B) IgA antibody responses against the five primary vaccine antigens in different age groups in Bangladesh. Adult data from the same protocol included for comparison. All participants received two doses of ETVAX with or without dmLT or two doses of placebo. Mucosal responses were measured in ALS specimens for adults and children aged 12–59 months and in ALS or faecal specimens for infants aged 6–11 months. ALS=antibodies in lymphocyte secretions. ETVAX=enterotoxigenic Escherichia coli vaccine.