TL;DR: A two-year Shigella outbreak that sickened 169 primates at a Wisconsin research center gave scientists rare immune samples that revealed how to design a broader Shigella vaccine.

A natural outbreak became a vaccine study

A diarrheal outbreak that began in December 2022 at the Wisconsin National Primate Research Center in Madison lasted for more than two years and produced 169 microbiologically confirmed cases of shigellosis among non-human primates, according to Ars Technica.

The scale and duration were unusual for such a facility, where periodic Shigella flares occur but seldom persist at that size, according to Ars Technica. Some staff members at the Madison facility also became ill during the event.

Investigators treated the episode as an opportunity to study immunity after natural infection. The team collected serum, plasma and defined immune-cell types from 41 infected animals at one week, two weeks and four to six weeks after detection, according to Ars Technica.

The resulting analysis appeared in Science Translational Medicine, with an online record dated Sept. 2, 2026, and news coverage dated Sept. 3, 2026. The paper was titled “Multifunctional antibody responses from a primate Shigella outbreak inform vaccine design,” according to the study abstract in Science Translational Medicine.

Why Shigella needs a vaccine

There is currently no approved vaccine for Shigella spp., which is described as a leading cause of diarrhea with increasing rates of antimicrobial resistance, according to the study abstract in Science Translational Medicine.

Shigella strikes more than 200 million people worldwide each year and kills more than 200,000, mostly children, according to Ars Technica. People who survive infection typically develop some protective immune responses, but protection tends to cover only a narrow range of the bacterial family because Shigella includes various species and serotypes.

Shigella strains are becoming more resistant to antibiotics, adding urgency to prevention, according to Ars Technica. Multiple vaccine-development efforts are underway, but researchers have lacked detailed molecular and structural information about the determinants of immunity, according to Ars Technica and the study abstract in Science Translational Medicine.

The authors wrote that their work “revealed unexpected features of the anti-Shigella antibody response in naturally infected [non-human primates] and represents a step toward the rational design of Shigella vaccine candidates,” according to Ars Technica.

What sickened the animals

Researchers collected 151 Shigella isolates for serotyping and performed whole-genome sequencing on 95 representative isolates, according to Ars Technica.

The outbreak was traced to two related clusters of Shigella flexneri, a species described as commonly striking captive non-human primates, according to Ars Technica. Human outbreaks are more often driven by Shigella sonnei, although both species can infect either type of primate, according to Ars Technica.

Of 90 isolates tested for antimicrobial resistance, 63 percent were multidrug resistant, according to Ars Technica. That resistance profile mirrors the broader concern about drug-resistant Shigella in human disease.

Most primate cases caused only mild diarrheal symptoms, while some infections were asymptomatic and some produced severe symptoms, according to Ars Technica. Infants were the hardest hit, a pattern described as similar to human infection.

Non-human primates, like humans, are natural hosts for Shigella, which helps explain why the pathogen can move through a primate facility once introduced, according to Ars Technica.

How the immune samples were used

The research team isolated monoclonal antibodies against candidate Shigella vaccine antigens using samples from the Shigella flexneri outbreak in the non-human primate research facility, according to the study abstract in Science Translational Medicine.

The sampling design followed individual animals over time rather than relying on a single time point. Blood-derived products and immune cells were taken at one week, two weeks and between four and six weeks after infection was detected, according to Ars Technica.

That longitudinal collection allowed comparison of early and later antibody responses. The researchers then dissected which bacterial structures the antibodies recognized and what functional effects followed binding, according to Ars Technica.

The Wisconsin National Primate Research Center noted on Sept. 3, 2026, that Science Translational Medicine had chosen the vaccine-design article as its featured article for September, confirming institutional interest in the findings.

O-antigen antibodies gained breadth

One focus was the O-antigen, the outermost component of lipopolysaccharide, known as LPS, which protrudes from the bacterial outer membrane, according to Ars Technica.

LPS is found on Gram-negative bacteria, a group that includes Escherichia coli, Salmonella and Shigella, and it serves as a structural component of the outer membrane, according to Ars Technica. Its exposed position makes the outer O-antigen portion a common immune target, but variation among Shigella serotypes can limit cross-protection.

The study found that antibodies targeting the Shigella O-antigen can undergo substantial affinity maturation of more than 10 percent to acquire broad cross-reactivity across S. flexneri serotypes, according to the study abstract in Science Translational Medicine.

Some of those antibodies appeared to have been refined through repeated exposures and developed the ability to recognize O-antigen from many different Shigella serotypes, according to Ars Technica. Strong binding to O-antigen could also trigger cascading reactions of plasma proteins that end with bacterial cells being disintegrated, according to Ars Technica.

That combination of breadth and bactericidal function is directly relevant to vaccine design because a useful Shigella vaccine must protect beyond a single serotype.

Attention turned to the secretion-system tip

The second major focus was the type III secretion system, abbreviated T3SS, a virulence-associated apparatus used by Shigella during infection, according to the study abstract in Science Translational Medicine.

When assembled, the T3SS forms a syringe-like structure with a needle extending from the bacterial cell into a target cell, according to Ars Technica. The bacterium then delivers proteins called effectors, which dampen host-cell defenses and help create conditions favorable to the invader, according to Ars Technica.

At the tip of Shigella’s T3SS are two proteins, IpaB and IpaD, which were attacked by antibodies from the infected primates, according to Ars Technica. The study found that the T3SS proteins IpaD and IpaB elicit moderate T-cell and robust antibody responses, according to the study abstract in Science Translational Medicine.

Those responses did not point in only one direction. T3SS antibodies could either inhibit or enhance bacterial virulence in laboratory assays and differed in animals depending on epitope specificity, according to the study abstract in Science Translational Medicine.

Protective versus harmful binding sites

The distinction between helpful and harmful antibodies depended on exactly where on the target the antibody attached. For IpaD, the regions targeted by neutralizing versus virulence-enhancing antibodies were mapped, providing guidance for selecting vaccine components that favor protective responses.

That epitope-level detail matters because including a whole protein in a vaccine could potentially elicit both kinds of antibodies. A design informed by the outbreak data could emphasize segments linked to inhibition and avoid segments linked to enhanced virulence.

The same principle applies to O-antigen work. Identifying the contact points where the most potent antibodies attached gives developers concrete molecular targets rather than a general instruction to include bacterial sugar, according to Ars Technica.

Together, the authors said, the findings provide insight into protective and deleterious immune responses against Shigella that directly inform vaccine immunogen design, according to the study abstract in Science Translational Medicine.

Who conducted the research

The listed authors include Robert M. Gallant, Paul Savarino, Sophia Pulido, Morgan S. A. Gilman, Ti Lu, Jennifer M. Hayes and colleagues, according to the study abstract in Science Translational Medicine.

Affiliations span the International AIDS Vaccine Initiative, Harvard University, the University of Missouri, the University of Wisconsin–Madison and the University of Cambridge, along with Latham Biopharm Group, according to the study abstract in Science Translational Medicine.

Funding included support from the National Institutes of Health, including programs for primate facilities and construction of extramural research facilities, according to the study abstract in Science Translational Medicine.

The Wisconsin center’s announcement identified the paper as work on vaccine design for Shigella and linked it to the September featured article in Science Translational Medicine.

What this changes for vaccine developers

The practical message is that O-antigen-based vaccines may be more feasible across serotypes than previously assumed if immunogens can be engineered to elicit the broadly cross-reactive type of response observed after natural infection.

The T3SS findings add caution alongside opportunity. Because antibodies to IpaB and IpaD showed both inhibitory and enhancing behavior tied to epitope specificity, future candidates will need to be evaluated for functional direction, not merely binding strength.

The study does not announce an approved vaccine or a clinical product. It reports naturally acquired antibody and T-cell features and describes them as information that can guide rational construction of Shigella vaccine candidates, according to the study abstract in Science Translational Medicine.

For a pathogen that still causes more than 200 million illnesses and more than 200,000 deaths annually, with 63 percent multidrug resistance in the 90 tested outbreak isolates, that design guidance is the central advance, according to Ars Technica and the study abstract in Science Translational Medicine.

Sources referenced