Small trial finds an oral Shigella vaccine shielded most who took it

An experimental two-dose oral vaccine protected 31 of 34 challenged adults in an early trial, but young children were not tested.

Researchers reported that an experimental oral vaccine called WRSs2 protected adults against shigellosis, the bacterial diarrheal disease caused by Shigella. The findings appeared June 30 in The Lancet Infectious Diseases. The vaccine uses a weakened strain of S. sonnei, given as two doses of 500,000 bacteria 28 days apart. There are currently no approved vaccines for the disease.

The phase 2 trial enrolled 108 US adults ages 18 to 49 with little preexisting immunity. In a controlled inpatient challenge with roughly 1,500 live bacteria, 3 of 34 vaccinated participants developed shigellosis versus 21 of 26 placebo recipients. Severe symptoms hit 1 vaccinated participant compared with 18 in the placebo group.

The stakes are large. Shigella causes an estimated 450,000 US infections a year, about 242,000 of them resistant to at least one drug, and at least 80 million infections worldwide. It is most dangerous for children under five and can be fatal. A vaccine that heads off infection would reduce both illness and the antibiotic use that breeds resistance.

The caveats are real. This was a small trial in adults, not the young children who bear the greatest burden. Two participants developed severe shigellosis and dropped out. The dominant global species is S. flexneri, not the S. sonnei tested, so worldwide effectiveness is unclear, and an outside expert said a confirmatory trial would take three to six years.

Source: Science News