Persistent Efficacy of Vi Conjugate Vaccine against Typhoid Fever in Young Children

Abstract

After 27 months of active surveillance in a double-blind, placebo-controlled, randomized trial involving 12 008 children (2-5 years of age) in the Mekong Delta region of Vietnam where it was found that a Vi conjugate vaccine (Vi-rEPA) against Salmonella typhi conferred protection in 91.1% of individuals, 5232 children that were initially assigned to the placebo group was given one injection of Vi-REPA. There was age-related persistence of IgG anti-Vi antibodies, with higher levels found in older children. Two to 20 weeks after cross-vaccination, the geometric level of IgG anti-Vi antibody in the newly vaccinated group was 38.14 ELISA units, similar to levels observed in children 5-14 years old in a different study.