Whole genome analysis to detect potential vaccine-induced changes on Shigella sonnei genome

Shigellosis or bacillary dysentery is endemic worldwide and is a significant cause of death in children less than five years of age in developing countries. There are no licensed Shigella vaccines and glycoconjugates are among the leading candidate vaccines against shigellosis today. We used whole genome sequence analysis (WGA) to find out whether immunization, with an investigational Shigella sonnei glycoconjugate, could induce selective pressure leading to changes in the genome of S. sonnei. An outbreak of culture-proven S. sonnei shigellosis which occurred immediately after vaccination in one of the cohorts of volunteers participating in a phase III trial of the vaccine in Israel created a unique condition in which the epidemic agent "co-existed" with the developing immune responses induced by the vaccine and natural infection among vaccinees who developed S. sonnei shigellosis. By comparing the whole genomes of S. sonnei isolated from vaccinees and from volunteers in the control group, we show at a very high sensitivity that a potent S. sonnei glycoconjugate that conferred 74% protective efficacy against the homologous disease did not induce changes in the genome of S. sonnei and in particular on the O-antigen gene cluster.