Method of immunizing humans against

S. typhi Pseudomonas aeruginosa S. typhi S. typhi S. typhi. This invention relates to conjugates of the Vi polysaccharide of with the carrier recombinant exoprotein A (rEPA), and compositions thereof, and to methods of using of these conjugates and/or compositions thereof for eliciting an immunogenic response in humans, including responses which provide protection against, or reduce the severity of, bacterial infections. The conjugates, and compositions thereof, are useful as vaccines to induce serum antibodies against and are useful to prevent and/or treat illnesses caused by