This Phase 1 clinical trial is aimed to evaluate the safety and immunogenicity of 3 doses of 5 sequentially escalating dosages of a candidate vaccine against Shigella sonnei (1790GAHB vaccine) administered by intramuscular route in healthy adults (18 to 45 years of age at enrollment). The safety profile of the 1790GAHB vaccine is evaluated in comparison to that of placebo (GAHB-Placebo), constituted by an aluminum hydroxide suspension having the same concentration as study vaccine formulations. A total of 50 eligible subjects will be assigned to one of five sequential cohorts of 10 subjects each.