This Phase 1 clinical trial is aimed to evaluate the safety and immunogenicity of 3 doses of a candidate vaccine against Shigella sonnei (1790GAHB vaccine) when administered at different dosages by different routes (intradermally, intranasally or intramuscularly) 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 52 eligible subjects will be assigned to one of three sequential cohorts as follows: