An experimental oral vaccine significantly reduced shigellosis infections in a Phase 2 clinical trial, offering hope for preventing a bacterial disease that causes millions of infections worldwide each year. The findings were published in The Lancet Infectious Diseases Researchers tested a vaccine called WRSs2, which contains a weakened strain of Shigella sonnei bacteria. In the trial, 108 US adults aged 18 to 49 with little pre-existing immunity received either two doses of the vaccine or a placebo, administered 28 days apart.
Nearly a month after vaccination, participants received a controlled oral dose of live, full-strength S. sonnei bacteria in a closely monitored inpatient setting. Only 3 of 34 vaccinated participants developed shigellosis, compared with 21 of 26 placebo recipients. Just 3% of vaccinated participants developed severe symptoms such as high fever or excessive diarrhoea, compared with 69% of those receiving placebo.
Study co-author Nadine Rouphael, a vaccinologist at Emory University, noted the protection level exceeds what has generally been reported in previous Shigella vaccine studies. Currently, no approved vaccines exist for shigellosis.
Shigellosis spreads through contaminated food, water, surfaces, or sexual contact. In the United States, the bacterium causes an estimated 450,000 infections annually, with approximately 242,000 resistant to at least one antimicrobial drug. Worldwide, infections exceed 80 million yearly, with children under five facing the greatest risk.
Experts noted limitations: the vaccine targeted S. sonnei, the dominant species in the United States, whilst S. flexneri predominates globally. Further trials in young children in low- and middle-income countries, who bear the greatest disease burden, will be needed. Regulatory approval could take three to six years.
Source: Science News / The Lancet Infectious Diseases (2026)
