Protecting travelers from typhoid fever.

Although the incidence of domestically acquired typhoid fever is declining, the proportion of cases resulting from foreign travel has continued to rise. The widely available heat-phenol-inactivated parenteral vaccine has an efficacy of 65% but evokes severe adverse reactions in approximately 25% of recipients. A major advance in the control of typhoid fever was the development of an oral live, attenuated vaccine. Three doses of Ty21a in an enteric coated formulation given over one week provides 69% efficacy for at least four years. A series of four doses confers maximum protection. Increasing the interval between doses does not improve protection. Vaccine should be refrigerated and is currently not recommended for pregnant women, children older than 6 years of age, or immunocompromised patients. With respect to vaccines that are not available in the United States, parenteral purified Vi polysaccharide is considered safe and provides 64% to 72% protection over 21 months. Lastly, the safety and immunogenicity of an auxotrophic (Aro-, Pur-) S typhi recently has been evaluated. Travelers to areas of high risk should be vaccinated but must be cautioned that vaccination is not a substitute for careful selection of food and water. Since typhoid vaccines are not 100% effective, the protection of the vaccine can be overcome by large inocula of S typhi. Physicians caring for world travelers must also keep in mind that other infections, such as plague, typhus, and arboviral fever, may mimic a typhoidal pattern.