E. coli Vaccine Fails to Prevent Invasive Infections

AN EXPERIMENTAL vaccine designed to prevent invasive Escherichia coli infections in older adults has failed to demonstrate clinical efficacy in a large Phase III trial, despite generating robust and sustained immune responses.

The E.mbrace trial, involving 17,701 participants across 20 countries, found no demonstrated protective benefit from the nine-valent ExPEC9V vaccine compared with placebo. Following a prespecified futility analysis, the study was terminated early, and development of the vaccine has been discontinued.

Addressing the Burden of Invasive E. coli Disease

Invasive E. coli disease (IED), caused by extraintestinal pathogenic E. coli (ExPEC), can lead to serious complications, including bloodstream infections and sepsis.

Older adults are particularly vulnerable to severe infections, highlighting the need for effective preventive strategies.

ExPEC9V was developed as a conjugate vaccine targeting nine E. coli O-serotypes responsible for a substantial proportion of bloodstream infections.

To evaluate its efficacy, safety, and immunogenicity, researchers conducted the E.mbrace trial, a randomised, double-blind, placebo-controlled Phase III study across 332 clinical sites in 20 countries.

The study enrolled adults aged 60 years and older who were in stable health and had experienced a urinary tract infection within the previous 2 years.

Participants were randomly assigned to receive a single intramuscular dose of ExPEC9V or placebo.

The primary endpoint was the first occurrence of invasive disease caused by one of the nine vaccine-targeted E. coli serotypes, identified in blood or other normally sterile sites.

Vaccine Fails to Demonstrate Protective Efficacy

Between June 2021 and February 2025, 17,701 participants were enrolled, randomised, and vaccinated, including 8,858 receiving ExPEC9V and 8,843 receiving placebo.

The per-protocol efficacy population comprised 17,637 participants, with 8,822 in the vaccine group and 8,815 in the placebo group.

During the study, 24 cases of invasive disease caused by vaccine-targeted serotypes were recorded.

Of these, 11 occurred among ExPEC9V recipients and 13 among placebo recipients.

The estimated vaccine efficacy was 14.63% (95% CI −106.57 to 65.38), demonstrating no statistically established protective effect.

The wide confidence interval reflected considerable uncertainty around the efficacy estimate, with the small number of infections limiting the precision of the findings.

A prespecified futility analysis, conducted after at least 40% of the anticipated 53 invasive disease events had occurred, indicated that the trial was unlikely to demonstrate the intended efficacy.

Consequently, the study was terminated before reaching its planned enrolment of 19,800 participants.

Strong Antibody Responses Did Not Translate Into Protection

Despite the absence of demonstrated clinical efficacy, ExPEC9V generated robust and sustained antibody responses against the targeted E. coli serotypes.

This finding highlights an important distinction between immunogenicity and clinical protection, demonstrating that strong vaccine-induced antibody responses do not necessarily translate into reduced infection risk.

The vaccine also demonstrated an acceptable safety profile during the study.

Low-grade adverse events were more frequent among participants receiving ExPEC9V than among those receiving placebo.

However, serious adverse events were similarly distributed between the two groups, and no vaccine-related deaths were reported.

These findings suggest that the vaccine was generally tolerated, although its safety profile did not compensate for the absence of demonstrated clinical benefit.

Implications for Future E. coli Vaccine Development

The E.mbrace findings represent a significant setback for efforts to prevent invasive E. coli disease through vaccination, particularly among older adults with a history of urinary tract infection.

Although ExPEC9V targeted nine clinically relevant serotypes and generated sustained antibody responses, these immunological findings were insufficient to establish protection against invasive infection.

The limited number of primary endpoint events and early trial termination also restricted the precision of the efficacy estimate. Nevertheless, the prespecified futility assessment supported the decision to discontinue the trial.

The researchers concluded that ExPEC9V did not demonstrate efficacy in preventing invasive disease caused by vaccine-targeted E. coli serotypes in older adults.

Development of ExPEC9V has subsequently been discontinued, underscoring the challenges of translating vaccine-induced immune responses into meaningful protection against invasive bacterial infections.

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