A team from Vietnam and the USA evaluated the safety, immunogenicity, and efficacy of the Salmonella typhi Vi conjugate vaccine in children 2 to 5 years old.
The newly devised vaccine is a conjugate of the capsular polysaccharide of Salmonella typhi , Vi, bound to nontoxic recombinant Pseudomonas aeruginosa exotoxin A (rEPA). It has enhanced immunogenicity in adults and in children 5 to 14 years old, and has elicited a booster response in children 2 to 4 years old.
A total of 11,091 children from 16 communities in the Dong Thap Province, Vietnam were enrolled in the double-blind, randomized trial.
Each subject received 2 injections, 6 weeks apart, of either Vi-rEPA or a saline placebo.
Cases of typhoid, diagnosed by the isolation of S. typhi from blood cultures after 3 or more days of fever (a temperature of 37.5°C or higher), were identified by active surveillance over a period of 27 months.
The researchers estimated efficacy by comparing the attack rates of typhoid in the vaccine group with that in the placebo's group.
|The efficacy of the Vi-rEPA conjugate vaccine is 92%.
|New England Journal of Medicine|
S. typhi was isolated from 4 of the 5,525 children who were fully vaccinated with Vi-rEPA and from 47 of the 5,566 children who received both injections of placebo. From these figures, the authors calculated the efficacy to be 92%.
Among the 771 children who received only one injection, there was 1 case of typhoid in the vaccine group and 8 cases in the placebo group.
Cases were distributed evenly among all age groups and throughout the study period. No serious adverse reactions were observed.
In all 36 children studied 4 weeks after the second injection of the vaccine, the researchers found that levels of serum IgG Vi antibodies had increased by a factor of 10 or more.
Dr Feng Ying C. Lin, of the National Institute of Child Health and Human Development, Bethesda, Maryland, concluded on behalf of the group, "The Vi-rEPA conjugate typhoid vaccine is safe and immunogenic and has more than 90% efficacy in children 2 to 5 years old.
"The antibody responses and the efficacy suggest that this vaccine should be at least as protective in persons who are more than five years old."