Combined vaccination against yellow fever and typhoid fever: a comparative trial.

In order to evaluate a combination of yellow fever and typhoid fever vaccine, we conducted a controlled trial comparing reactogenicity and immunogenicity of Vi polysaccharide (ViPS) vaccine and yellow fever 17D (YF) vaccine after single, simultaneous and combined administration. The combined YF/ViPS vaccine was prepared by using the liquid ViPS vaccine as a diluent for the YF vaccine. The stability of such a reconstitution had been assessed in vitro. Safety was evaluated using a self-surveillance form and by repeated clinical visits. Immunogenicity was evaluated by a plaque reduction test for YF and by radioimmunoassay for ViPS. Tolerability was satisfactory in all groups. There was no increase in local or general reactions in groups receiving both vaccines, whether given simultaneously or combined. The serological response to ViPS was similar after single and simultaneous or combined administration. Interestingly, the immune response to YF was significantly enhanced in groups receiving the vaccines simultaneously or combined, suggesting a potential adjuvant effect of ViPS.