Long Term Protection by and Persistence of Vi Antibodies Induced by Vi-rEPA Conjugate Vaccines in Vietnamese Children Injected at 2-5 Years or at 5-8 Years of Age

Typhoid fever remains an important cause of morbidity and mortality in the developing world. It is estimated that more than 16 million cases and about 600,000 deaths occur annually, most of which occur in Southeast Asia and Africa. Ingestion of food or water contaminated by acutely infected persons or chronic carriers is the most common form of transmission. As a result, typhoid fever is prevalent where unsafe drinking water or contaminated food is common. Typhoid fever is highly endemic in Vietnam, especially in the southern provinces, and is a significant disease in both preschool and school-aged children. Data from Dong Thap Provincial Hospital, Mekong delta region showed that among 3,934 hospitalized typhoid fever cases from 1990 to 1995, 4.2% had complications and 0.8% died. Typhoid fever has become difficult and expensive to treat. About 90% of Salmonella typhi isolates are of multidrug-resistance (resistant to chloramphenicol, ampicillin, and trimethoprim-sulfamethoxazole) and 76% of isolates showed reduced susceptibility to fluoroquinolones. Isolates with full fluoroquinolone or extended spectrum cephalosporin resistance have not been reported yet in Vietnam but occur sporadically in the Indian subcontinent. If they become widespread, alternative treatment options will be limited. The improvement of sanitation, provision of safe water and elimination of chronic carriage are not expected to be achieved quickly. Accordingly, vaccination against typhoid fever is an increasingly important national public health priority.