IMMUNOGENICITY, EFFICACY AND SEROLOGICAL CORRELATE OF PROTECTION OF SALMONELLA-TYPHI VI CAPSULAR POLYSACCHARIDE VACCINE 3 YEARS AFTER IMMUNIZATION

The protective efficacy and immunogenicity of Vi capsular polysacchari de vaccine against typhoid fever was measured 3 years after its admini stration 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 antibodie s than controls, as measured by radio-immunoassay (GMT 1.28 vs 0.76 mu 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 an d the relative ratio of antibody levels. The estimated protective leve l is I mu g ml(-1) suggesting that at a mean age of 9 years, 64% of va ccinates and 40% of controls had protective antibody against typhoid f ever in this endemic area. Copyright (C) 1996 Elsevier Science Ltd.