Recent research on vaccines and prevention of typhoid fever

Abstract

The spread of multi-resistant strains of S. typhi in the 1990s has invigorated interest in typhoid vaccines as tools to assist in the control of typhoid fever. Target populations for typhoid vaccines for typhoid vaccines include children in developing countries, travelers and clinical microbiologists. During the 1980s, two new typhoid vaccines were licensed in many countries, including Salmonella typhi Tylla as a live oral vaccine and purified Vt capsular polysaccharide as a parenteral vaccine. In controlled field trials, these vaccines conferred a moderate level of protection for 3-7 years. Whereas the well-tolerated Ty21a and Vi vaccines replaced the old reactogenic killed whole-cell parenteral vaccines in travellers, they were not readily adopted by developing countries for control of endemic typhoid. Health authorities in developing countries contend that they lack the resources to administer Tylla and Vi to school children, the age group in which these vaccines are known to be protective. Rather, they express interest in typhoid vaccines that could be administered to infants 0-12 montlis of age through the Expanded Program on Immunization. Since neither Vi nor Tylla is satisfactorily immunogenic in infants, more immunogenic vaccines are being sought. One highly promising approach consist of Vt conjugates in which Vt, after being covalently bound to a carrier protein such as tetanus toxoid, elicits significantly higher titers of Vi antibody and stimulates immunologic memory. In another approach, three new genetically-engineered strains of S. typhi, CVD 908-htrA, Ty800 and x 4073, offer promise as single-dose live oral vaccines. It is anticipated that these new vaccines will be markedly more immunogenic in infants. If they confer long-term protection enduring through school age, they may serve as public health tools to control typhoid in endemic areas.