shigella flexneri-shigella sonnei bivalent conjugate vaccine

A **bivalent conjugate vaccine** targeting *Shigella flexneri* (typically serotype 2a) and *Shigella sonnei* combines O-antigen polysaccharides from both bacterial strains, chemically conjugated to a carrier protein (often recombinant exotoxin A of *Pseudomonas aeruginosa* [rEPA] or similar)[1][2]. The purpose is to elicit strong, long-lasting immune responses (notably high titers of IgG antibodies against the O-antigens) to protect against moderate to severe diarrheal disease or dysentery caused by these two most prevalent *Shigella* serotypes. The vaccine is under clinical development, with field efficacy trials indicating immune response and significant protection in adults and children, though efficacy in children under age 3 is less established[2]. Primary indications are for the prevention of shigellosis in children in endemic areas and in adults at high risk (travelers, military personnel)[1][2]. Multiple candidates exist, including O-antigen conjugates to rEPA and bioconjugates[2].