Researchers reported results of a clinical trial for the WRSs2 vaccine on June 30 in The Lancet Infectious Diseases. Three out of 34 participants who received two doses of the vaccine developed shigellosis, while twenty-one out of 26 participants who received the placebo developed shigellosis.

The WRSs2 vaccine is an oral vaccine containing a weakened strain of the Shigella species S. sonnei. The clinical trial enrolled 108 U.S. adults ages 18 to 49 who had little preexisting immunity to S. sonnei. Two participants developed symptoms of severe shigellosis and did not continue in the trial.

A group of participants received two doses of 500,000 weakened S. sonnei bacteria 28 days apart, and another group of participants received a placebo. Nearly a month after vaccination, participants received a live, full-strength oral dose of roughly 1,500 S. sonnei bacteria in a monitored inpatient setting.

Eighteen placebo recipients, or 69 percent, developed severe shigellosis symptoms such as high fever or excessive diarrhea. "The protection is higher than what has generally been reported for prior Shigella vaccine studies," Nadine Rouphael, vaccinologist at Emory University, said.

"The vaccine has to work in the gut, where the infection starts, and it must be strong enough to protect but gentle enough not to cause too many stomach and intestinal symptoms," Rouphael said. She added that she would like to test it in populations that need it most, especially young children in places where Shigella is common.

Currently, there are no approved vaccines for shigellosis. Shigellosis spreads when tiny amounts of fecal matter contaminate food, water, or surfaces, and can also spread through sexual contact. In the United States, Shigella causes an estimated 450,000 infections each year.

About 242,000 Shigella infections in the United States each year are resistant to at least one antimicrobial drug. Worldwide, Shigella causes at least 80 million infections each year.

Shigellosis symptoms include diarrhea, fever, nausea, and abdominal cramps. Shigellosis is most dangerous for children under age 5 and can be deadly. The vaccine tested S. sonnei, which is the dominant Shigella species in the United States, while S. flexneri is the dominant species worldwide.

The results indicate a potential advance in preventing a disease that causes significant illness globally, though experts note substantial hurdles remain before any product reaches the market. "It’s a long way from being approved by vaccine regulatory authorities, as a big clinical trial designed to confirm effectiveness would take three to six years to complete," Shangxin Yang, molecular microbiologist at UCLA School of Medicine, said. Yang noted that because the vaccine was tested on adults and on the less common global species, it’s unclear whether this vaccine would work in young children in low- and middle-income countries, who bear the greatest disease burden.

Other researchers pointed to alternative approaches in development. "Finding the right oral dose of the vaccine for the general population might be difficult, as different groups could react to it differently," David Sack, epidemiologist at Johns Hopkins Bloomberg School of Public Health, said. Sack was part of a team working on an intramuscular vaccine that reported a 30 percent reduction in shigellosis. "Other injectable vaccines are also showing promise and could probably be safer," Sack said.