The aim of this double-blind randomised vaccine-controlled trial was to assess the efficacy of a conjugate vaccine composed of Shigella sonnei O-specific polysaccharide bound to Pseudomonas aeruginosa recombinant exoprotein A (S sonnei-r EPA) and of an oral, live-attenuated Escherichia coli/S flexneri 2a (EcSf2a-2) hybrid vaccine among military recruits in Israel at high risk of exposure to Shigella spp. We report here our preliminary findings on the efficacy of S sonnei-rEPA; we have not documented sufficient cases to assess the efficacy of EcSf2a-2.

Between April, 1993, and August, 1994, male Israeli military recruits aged 18–22 years were asked to take part in our study. We enrolled 1446 soldiers from seven separate field sites (groups A–G). Soldiers were randomly allocated one injection of S sonnei-rEPA and four doses of oral placebo (n=576), four oral doses of EcSf2a-2 and one injection of saline placebo (n=580), or one injection of meningococcal tetravalent control vaccine and four doses of oral placebo (n=290). Because there were no cases of S flexneri 2a, the EcSf2a-2 and meningococcal vaccines were the control group. We defined S sonnei shigellosis as diarrhoea with a positive faecal culture for S sonnei. Each group of soldiers was followed up for 2·5–7·0 months. The primary endpoint was protective efficacy of S sonnei-rEPA against S sonnei shigellosis.

Cases of culture-proven S sonnei shigellosis occurred in four groups of soldiers (groups A–D), which comprised 787 volunteers (312 received S sonnei-rEPA, 316 received EcSf2a-2, and 159 received meningococcal control vaccine). In groups A–C, cases of shigellosis occurred 70–155 days after vaccination, whereas in group D cases occurred after 1–17 days. In groups A–C, the attack rate of shigellosis was 2·2% in recipients of S sonnei-rEPA compared with 8·6% in controls (protective efficacy 74% [95% CI 28–100], p=0·006). S sonnei-rEPA also showed significant protection against shigellosis in group D (43% [4–82], p=0·039). Prevaccination and postvaccination ELISA measurements of antibody to S sonnei lipopolysaccharide among recipients of S sonnei-rEPA showed that the vaccinees who developed S sonnei shigellosis had significantly lower serum IgG and IgA responses to the homologous lipopolysaccharide than those who did not (p=≤0·05).

One injection of S sonnei-rEPA confers type-specific protection against S sonnei shigellosis. The high antibody concentration induced by the conjugate vaccine in volunteers who did not develop shigellosis suggests that there is an association between serum antibody titre and protection.