No vaccine for Shigella has been approved anywhere in the world. Several candidates are in clinical trials, with the most advanced ones in Phase 2 testing, but a widely available vaccine is unlikely before the early 2030s. That timeline matters because Shigella causes an estimated 270 million episodes of diarrhea globally each year and kills between 28,000 and 64,000 children under five, nearly all in low- and middle-income countries.
Why a Vaccine Has Been So Hard to Develop
The core problem is diversity. The Shigella genus includes four species and more than 50 serotypes. The immune system primarily recognizes Shigella by a sugar chain on the bacterium’s outer surface called the O-antigen, and that chain differs from one serotype to the next. Immunity from surviving one Shigella infection, or from a vaccine targeting one serotype, does little to protect against others. Any effective vaccine needs to cover at least four of the most common serotypes to be useful in the real world.
Antibiotic resistance adds urgency. In a 12-year surveillance study of Shigella isolates in Shanghai, 98% of samples were resistant to multiple drugs. Resistance to several first-line antibiotics exceeded 90%, and resistance to azithromycin, one of the go-to treatments, climbed from 12.5% to 60% over the study period. Similar resistance patterns have been documented in Iran, Bangladesh, and the United States. A vaccine would reduce the need for antibiotics in the first place.
Vaccine Candidates in Clinical Trials
The most clinically advanced approach uses bioconjugate vaccines, which attach pieces of Shigella’s O-antigen sugar chain to a carrier protein. This trains the immune system to recognize the bacterium without exposure to live Shigella. The lead candidate along this path, called Flexyn2a, targets a single serotype (S. flexneri 2a). In a controlled human challenge study at Johns Hopkins University, 67 volunteers received either the vaccine or a placebo and were then deliberately exposed to Shigella. The vaccine reduced moderate-to-severe disease by about 52%, and among those who still got sick, vaccinated participants had significantly lower severity scores and were half as likely to need early antibiotic treatment.
Building on those results, a four-component version called S4V covers S. flexneri 2a, 3a, and 6, plus S. sonnei, the four most globally prevalent serotypes. It is currently in Phase 2 testing, including an age-descending dose-finding study in Kenya designed to work toward approval in the children who need it most.
A separate line of research takes a different approach entirely. Instead of targeting the O-antigen, some vaccine developers focus on proteins that Shigella uses to invade human cells. These invasion proteins are shared across serotypes, so a vaccine based on them could potentially protect against all Shigella species at once. Early-stage candidates using this strategy have shown broad protection in preclinical studies, though they are further from approval than the bioconjugate vaccines.
A third approach uses synthetic sugar chains that mimic the O-antigen. One such candidate, SF2a-TT15, uses a lab-built version of the S. flexneri 2a sugar chain linked to a tetanus carrier protein and is currently being evaluated for efficacy.
Who Would Get Vaccinated First
The World Health Organization has identified infants starting at 6 months and children up to 36 months as the primary target population. This age group bears the heaviest burden of severe Shigella disease and death, particularly in sub-Saharan Africa and South Asia. WHO has also noted interest in data showing protection lasting through age five, which would influence how broadly vaccination programs roll out.
Travelers are another group that would benefit. Shigella is a well-known cause of traveler’s diarrhea, with S. sonnei accounting for about two-thirds of travel-associated cases and S. flexneri roughly a quarter. Military personnel deployed to endemic regions have historically been at high risk as well. No travel-specific Shigella vaccine is available today, though the candidates in development could eventually fill that gap.
How Approval Could Happen
WHO has worked with regulators to map out two possible paths to approval. The traditional route requires a large field efficacy trial in children under five in endemic countries, which takes years and significant investment. An accelerated route would use data from controlled human challenge studies (where volunteers are deliberately infected in a clinical setting) to support an earlier conditional approval, with field data following later. Both pathways are being actively discussed, and the choice will depend on how Phase 2 results look.
Gavi, the Vaccine Alliance, projects that a Shigella vaccine is unlikely to be widely available until the early 2030s. That estimate reflects the time still needed for Phase 3 trials, regulatory review, manufacturing scale-up, and introduction into national immunization programs. For now, prevention relies on clean water, handwashing, and safe food handling, with antibiotics reserved for severe cases where resistance patterns still allow effective treatment.