For the first time, an O-specific polysaccharide antigen that is a Shigella sonnei, phase I, exopolysaccharide has been produced and characterized, said exopolysaccharide being an authentic natural compound in the form of a bacterial capsular polysaccharide. The exopolysaccharide contains a non-toxic lipid component, namely non-hydroxylated fatty acids, and exhibits low pyrogenicity and high immunogenicity. Effective, highly specific and safe vaccines for the prophylaxis and/or treatment of Shigella sonnei shigellosis are developed on the basis of the above-mentioned exopolysaccharide, as well as pharmaceutical compositions with a broad spectrum of action, in particular, in modulating immune response.