ETEC vaccine misses primary goal in Gambian infants but cuts severe diarrhoea

Enterotoxigenic Escherichia coli — ETEC — is one of the leading bacterial causes of childhood diarrhoea, responsible for 75 million diarrhoea episodes and up to 42,000 deaths a year in children, yet there is no licensed vaccine against it [s1]. A phase 2b trial in The Gambia has now tested the most advanced candidate, ETVAX, in exactly the age group that matters most. The headline result is a miss — but the detail underneath is more encouraging than the primary number alone suggests [s1].

The vaccine and the trial

ETVAX is an oral, inactivated, whole-cell ETEC vaccine, combined with a toxoid and a double-mutant heat-labile toxin adjuvant [s1]. The trial was double-blind and placebo-controlled, enrolling Gambian children aged 6-18 months from four centres and block-randomising them 1:1 to receive ETVAX or placebo on days 1, 15 and 90 [s1].

The primary efficacy endpoint was vaccine efficacy against moderate-to-severe ETEC-positive diarrhoea — MSD-ETEC — with co-infections involving Cryptosporidium, norovirus genogroup II, rotavirus or Shigella excluded, assessed in the per-protocol population [s1]. That strict definition, isolating diarrhoea attributable to ETEC alone, is what the trial was powered to answer.

Between Feb 22, 2021, and June 24, 2022, researchers screened 5253 children and randomly assigned 4936 — 2499 (51%) girls and 2437 (49%) boys — with 2468 to ETVAX and 2468 to placebo [s1].

What it found

Safety was clean. Serious adverse events occurred in 24 (1.0%) of 2474 children in the vaccine group and 32 (1.3%) of 2462 in the placebo group, with none related to the investigational product [s1]. In the immunogenicity subset of 122 children, ETVAX raised antibodies to the ETEC colonisation factors CFA/I and CS3 and to heat-labile toxin [s1].

On efficacy, the primary endpoint was not met. Vaccine efficacy against MSD-ETEC in the strict per-protocol analysis was 26.6%, with a confidence interval spanning zero (95% CI -58.3 to 66.0; p=0.43) [s1]. A result that wide cannot be distinguished from no effect.

But the secondary and exploratory analyses pointed the other way. Against MSD-ETEC regardless of copathogens, efficacy was 48.2% (p=0.053) [s1]. When only enteroparasitic copathogens — Giardia and Cryptosporidium — were excluded, efficacy reached 80.6% (p=0.0092) [s1]. And when the first dose was given before age 9 months, efficacy against all MSD-ETEC was 67.8% (p=0.026) [s1]. Against moderate-to-severe diarrhoea of any cause, ETVAX showed 21.4% efficacy (p=0.032) [s1].

How to read a missed primary endpoint

A trial that misses its pre-specified primary endpoint has, by the rules of evidence, not proven what it set out to prove — and the investigators say as much, concluding that ETVAX was safe and immunogenic but that the primary endpoint was not met [s1]. The supportive signals in the secondary and exploratory analyses are hypothesis-generating, not confirmatory, because the more impressive numbers come from definitions chosen after the fact or from subgroups.

What the pattern does suggest is that parasitic co-infections may have diluted the measurable benefit against ETEC, and that dosing earlier in infancy may matter [s1]. The authors' reading is that the findings suggest ETVAX protects Gambian children, even though the primary analysis fell short [s1].

The dilution problem is a recurring theme in childhood diarrhoea trials. In settings where several pathogens circulate at once, a single episode of diarrhoea may carry more than one organism, so a case counted as "ETEC" may in fact be driven by a parasite a vaccine cannot touch — which pulls an efficacy estimate toward zero even if the vaccine works against its target. That is why the exploratory analysis stripping out Giardia and Cryptosporidium produced a far larger effect, and why the investigators treat the co-pathogen question as central rather than incidental [s1]. It also complicates licensing: a regulator must decide whether to credit protection against ETEC specifically or against the diarrhoea a child actually experiences, which is rarely attributable to one cause alone.

Why the stakes are high

Diarrhoeal disease is the third leading cause of death in children aged 1-59 months and kills around 443,832 children under 5 each year, according to WHO, which counts nearly 1.7 billion cases of childhood diarrhoeal disease annually and ranks diarrhoea a leading cause of malnutrition in young children [s2]. A vaccine that could blunt one of the biggest bacterial contributors would be a genuine tool — which is why a near-miss in a well-run trial is treated as a reason to refine, not abandon.

What to watch

Whether the next trial pre-specifies the endpoints that looked strongest here — efficacy regardless of copathogens, and a dosing schedule that starts before 9 months — so that today's exploratory signals can be tested as hypotheses rather than discovered after the fact [s1]. On the evidence so far, ETVAX is safe and immunogenic in the children who need it most; whether it is effective enough to license remains, for now, unproven.

This article is informational and does not constitute medical advice.

Sources

• Efficacy of ETVAX, a vaccine against enterotoxigenic Escherichia coli-positive diarrhoea in Gambian children: a double-blind, randomised, placebo-controlled, phase 2b trial — The Lancet Infectious Diseases, 16 February 2026
• Diarrhoeal disease — Fact sheet — World Health Organization