A promising vaccine candidate for Shigella has demonstrated high levels of protection in a recent phase 2 clinical trial. The findings were published last week in The Lancet Infectious Diseases.
This challenge trial, conducted at two U.S. locations, involved 73 healthy adults between the ages of 18 and 49. Participants were randomly assigned to receive either WRS2, an oral vaccine designed to be live-attenuated and target Shigella sonnei, or a placebo. S. sonnei is one of four highly contagious species of bacteria that cause tens of millions of infections globally, leading to symptoms ranging from mild to severe diarrhea and vomiting. Currently, there are no approved vaccines for any Shigella species. Notably, two species, S. sonnei and S. flexneri, are showing increasing resistance to common antibiotics.
The study’s main goal was to track the incidence of shigellosis after participants were intentionally exposed to S. sonnei. This assessment took place between day 57 and day 63 post-challenge, focusing on those who received two doses of the vaccine at different levels and the placebo group. Researchers also monitored for any systemic adverse events.
Estimated Vaccine Efficacy Reaches 89%
Among the 34 participants who received the pooled two-dose vaccine regimen, only three (9%) developed shigellosis. In contrast, 21 out of 26 placebo recipients (81%) contracted the illness. This resulted in an estimated vaccine efficacy of 89%. Participants in both two-dose vaccine groups exhibited a robust antibody response. While most reported side effects were mild, six vaccine recipients experienced grade 3 adverse events. These more severe reactions prompted a review by the data safety monitoring board and led to a reduction in the vaccine’s dosage.
Moving forward, the research team plans to refine the optimal dose level and launch larger trials in regions where shigellosis is prevalent. These future studies will also include children, who are disproportionately affected by shigellosis in high-burden areas like sub-Saharan Africa.
Dr. Robert W. Frenck, Jr., senior author of the study and director of the Vaccine Research Center at Cincinnati Children’s Hospital, stated in a news release that the study marks a significant stride in developing a safe and effective vaccine against Shigella. He added, "With continued research, we have the potential to significantly reduce the global burden of this disease, particularly among children."