orally or parenterally, and 3 weeks later the vaccinated and control calves were challenged orally with pathogenic S. typhimurium (Robertsson et al., 1983). The oral attenuated vaccine protected significantly better than the parenteral killed vaccine. These results are sufficiently promising to evoke interest in analogous aromatic-dependent S. typhi oral vaccine strains for human use.

New vaccines to protect against typhoid fever, particularly the live oral vaccines, provide an opportunity for the disease control in endemic areas. However, further vaccine studies are impeded by the lack of understanding of the immunological basis of protection and the lack of suitable animal models for typhoid fever. At this point, the only way to assess the efficacy of a vaccine is through large-scale field trials in endemic areas.

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