Safety and immunogenicity of the oral E. coli K12-S. flexneri 2a vaccine (EcSf2a-2) among Israeli soldiers

A double-blind placebo-controlled study was carried out on the safety and immunogenicity of the oral Shigella flexneri (EcSf2a-2) vaccine among Israeli soldiers. Sixty volunteers received the vaccine and 59 received placebo. Fifty-three were given the full vaccine regimen (four doses). Doses ranged between 4.1 × 10⁸ and 1.1 × 10⁹ c.f.u. Visits to the unit clinic for mild gastrointestinal symptoms were common after the first dose in vaccinees (13%) as compared with placebo recipients (5%), but the difference was not significant, p = 0.12. Similarly, there was no difference between the groups for either gastrointestinal or non-gastrointestinal complaints reported by questionnaire. The vaccine strain was excreted by 69% and 67% of the vaccinees one day after receiving the second and the fourth doses, respectively. As judged by antibiotic susceptibility, phage typing and restriction fragment length polymorphism (RFLP), the vaccine strain emerged as genetically stable after replication in human gut and shedding. There was neither bacteriological nor serological evidence of transmission of the vaccine from vaccinees to placebo recipients. Eighteen of 26 (69.2%) and 11 of 30 (36.7%) vaccinees had significant IgA secreting cell responses 7 and 21 days after the first dose, respectively. Significant IgA or IgG serum antibody response to S. flexneri 2a LPS was detected in 30% of the vaccinees. These results support further evaluation of EcSf2a-2 vaccine protective efficacy in field studies.