Can a vaccine finally stop Shigella, the bug behind severe diarrhea?
Yes. In this trial, two doses of a Shigella vaccine called WRSs2 gave 89 percent protection against shigellosis when healthy adults were deliberately exposed to the germ. This was a phase 2, double-blind, randomised, placebo-controlled trial in adults at two sites in the USA, using a controlled human infection model.
Shigella is a type of bacteria that causes a gut infection called shigellosis, and it is a leading cause of bacterial diarrhea and dysentery. Despite long-standing research, there is still no licensed vaccine against it. This new study suggests that is starting to change.
What the researchers tested
The researchers used a setup called a controlled human infection trial. In plain terms, healthy volunteers agreed to get either the real vaccine or a placebo, and then were purposely given the Shigella germ under close medical watch. This kind of study sounds extreme, but it is one of the fastest and clearest ways to see if a vaccine truly works.
The vaccine, WRSs2, is live-attenuated. That means it uses a weakened version of Shigella sonnei to teach the immune system how to fight the germ. It had already shown safety and immunogenicity in earlier work. Doses were given 28 days apart, and volunteers were challenged orally with Shigella sonnei 28 days after the second vaccination.
What the data show
A total of 108 healthy adults aged 18 to 49 were enrolled between October 11, 2022 and January 9, 2024. They were split across four groups: 22 received two doses at the original dose level, 26 received two doses at a reduced dose level, 23 received one placebo dose followed by one dose of WRSs2, and 37 received placebo only. Of the 108, 73 went on to the challenge phase, where they were exposed to live Shigella sonnei under supervision: 16, 18, 13, and 26 participants from those respective groups.
The results were striking. Adjudicated shigellosis occurred in 3 of 34 participants (9 percent) who had received two doses of vaccine, compared with 21 of 26 placebo recipients (81 percent). That works out to a vaccine efficacy of 89 percent, with a 95 percent confidence interval of 71 to 96 percent.
Safety was not a clean sweep. Six participants had grade 3 adverse events after vaccination, which triggered two data and safety monitoring board reviews. After the first, the protocol was amended to lower the vaccine dose and tighten eligibility criteria. No change followed the second review. There were no vaccine-related serious adverse events and no deaths.
Dr. Kumar’s Take
I find this encouraging. Shigella is a stubborn problem and there is no licensed vaccine for it, so an 89 percent efficacy figure in a human challenge trial is a genuinely meaningful result. Three cases out of 34 vaccinated volunteers versus 21 out of 26 on placebo is a large, unambiguous gap.
That said, I want to be clear about what this is and is not. This was a phase 2 trial in healthy adults aged 18 to 49, and only 73 people reached the challenge stage. Six people had grade 3 reactions after vaccination, enough to prompt a dose reduction mid-trial, and the investigators themselves say further optimisation is needed to define the right balance between safety and efficacy. The people who need this vaccine most were not the ones tested here. Encouraging early results do not always hold up in larger, more diverse groups. So I am optimistic, but I am watching for the bigger trials that come next.
How the study was done
The trial used a double-blind, randomized design with site-stratified permuted-block assignment. Randomized means people were sorted into the vaccine or placebo group by chance, which keeps the two groups fair and comparable. Double-blind means neither the volunteers nor the researchers knew who got what, which stops expectations from coloring the results. The original design allocated participants 1:1:1 across two-dose vaccine, one-dose vaccine, and placebo. After 69 participants were enrolled, the safety review and protocol amendment shifted subsequent participants to a 2:1 split between a lower-dose two-dose vaccine group and placebo.
The primary endpoint was shigellosis as adjudicated by an endpoint review committee, and safety was assessed in everyone who was vaccinated. Pairing that rigorous design with a controlled human infection model gives the findings real weight. Instead of waiting years to see who catches Shigella in the community, the researchers could measure protection directly and quickly. The main trade-off is size. A challenge trial like this is small and tightly controlled by nature, so the numbers, while strong, come from a limited group.
Practical Takeaways
- There is no licensed Shigella vaccine available yet, so WRSs2 is still experimental and not something you can ask your doctor for today.
- If you travel to areas where Shigella is common, the practical protections remain careful hand washing, safe drinking water, and clean food handling.
- Watch for larger trials, since those results will decide whether this vaccine moves toward licensing.
- If you develop bloody diarrhea with fever after travel, see a doctor promptly.
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FAQs
What is shigellosis and how serious is it?
Shigellosis is a gut infection caused by Shigella bacteria, and Shigella is a leading cause of bacterial diarrhea and dysentery worldwide. In this trial, 21 of 26 placebo recipients developed adjudicated shigellosis after being exposed to the germ, which shows how readily it takes hold once someone is exposed. The volunteers here were healthy adults under medical supervision; the illness is a far bigger threat in settings where care is harder to reach.
Is it safe to deliberately infect people with Shigella in a study?
Controlled human infection trials are done under tight medical supervision, which is what makes them workable. Volunteers are healthy adults who give informed consent, and doctors monitor them closely. In this trial, participants were challenged orally 28 days after their second vaccination, and safety was assessed in every vaccinated participant. Six people had grade 3 adverse events after vaccination, which prompted two independent safety reviews and a dose reduction. No vaccine-related serious adverse events and no deaths occurred.
When could a Shigella vaccine actually be available?
That is still unknown. WRSs2 performed well in this phase 2 trial, but there is no licensed Shigella vaccine yet, and the investigators frame their findings as support for further clinical development of live-attenuated Shigella vaccines rather than as a finish line. They also note that further optimisation is needed to better define the safety and efficacy balance.
Bottom Line
In a controlled human infection trial in the USA, two doses of the live-attenuated vaccine WRSs2 gave 89 percent protection against Shigella sonnei shigellosis, with 3 cases among 34 vaccinated participants versus 21 among 26 placebo recipients. Six participants had grade 3 adverse events after vaccination, prompting a mid-trial dose reduction, though no vaccine-related serious adverse events or deaths occurred. The findings come from a phase 2 study of 108 healthy adults aged 18 to 49, and they support further clinical development of live-attenuated Shigella vaccines.
