Typhim Research Articles

Immunogenicity, efficacy and serological correlate of protection of Salmonella typhi Vi capsular polysaccharide vaccine three years after immunization

The protective efficacy and immunogenicity of Vi capsular polysaccharide vaccine against typhoid fever was measured 3 years after its administration in a double-blind randomized trial. Vaccine efficacy was not significantly different during each year of the trial and was 55% (95% CI: 30–71%) over the 3 year period. In a case-control study at 3 years after vaccination, recipients of Vi had higher levels of Vi antibodies than controls, as measured by radio-immunoassay (GMT 1.28 vs 0.76 μ g ml −1, P=0.0004) and by passive haemagglutination assay (GMT 10.46 vs 3.52, P=0.0001). The serological correlate of protection has been estimated using the relative risks of typhoid fever in the 2 groups and the relative ratio of antibody levels. The estimated protective level is 1 μg ml −1 suggesting that at a mean age of 9 years, 64% of vaccinates and 40% of controls had protective antibody against typhoid fever in this endemic area.

Experience with Vi typhoid capsular polysaccharide vaccine in the U.K.

Adult volunteers were immunised with a single dose of typhoid Vi capsular polysaccharide vaccine. After immunisation, 96% of the 103 subjects seroconverted and 94% had antibody levels above the protective threshold. Systemic reactions were uncommon, local reactions were mild and transient.