Full Breakdown
Oral Vaccine Shows Strong Protection Against Shigellosis in Early Human Trial
7/21/2026, 9:11:46 PM
Promising Results from a Small Oral Shigella Vaccine Trial
Researchers reported on June 30 that two oral doses of the experimental vaccine WRSs2 protected participants from shigellosis, an infection caused by Shigella bacteria. The study, published in *The Lancet Infectious Diseases*, is the first to demonstrate efficacy higher than that reported in prior Shigella vaccine attempts. No vaccine for shigellosis is currently approved in the United States or elsewhere.
Trial Design, Participant Numbers, and Efficacy
The trial enrolled 108 U.S. adults aged 18-49 who had little pre-existing immunity to *Shigella sonnei*. Two participants withdrew after developing severe shigellosis before receiving the study vaccine. The remaining volunteers received up to two oral doses of 500,000 weakened *S. sonnei* bacteria, spaced 28 days apart, while a control group received placebo.
Approximately one month after the dosing schedule, participants were challenged with a live, full-strength oral dose of about 1,500 * S. sonnei* bacteria in a monitored inpatient setting. Among the 34 vaccinated participants, only 3 developed shigellosis (? 9 %). By contrast, 21 of 26 placebo recipients (? 81 %) became ill. Severe symptoms—high fever or excessive diarrhea—occurred in 1 vaccinated individual (3 %) versus 18 placebo recipients (69 %).
Expert Interpretation and Remaining Questions
Molecular microbiologist Shangxin Yang (UCLA) called the results “highly encouraging” but emphasized that a large confirmatory trial could require three to six years.
Epidemiologist David Sack of the Johns Hopkins Bloomberg School of Public Health warned that identifying the optimal oral dose for broader populations may be difficult, noting that different groups could react variably. He also pointed out that the trial targeted the dominant U.S. species *S. sonnei*, whereas the globally prevalent species is *S. flexneri*. Consequently, the vaccine’s effectiveness in young children in low- and middle-income countries—who bear the greatest disease burden—remains uncertain.
Next Steps and Potential Public Health Impact
If larger trials confirm efficacy, the vaccine could address an estimated 450,000 U.S. infections annually—about 242,000 of which are resistant to at least one antimicrobial drug—and the worldwide burden of at least 80 million cases each year.