Comparison of safety and immunogenicity of a Vi polysaccharide typhoid vaccine with a whole-cell killed vaccine in Malaysian Air Force recruits

Objective. To carry out a comparative study of the safety and immunogenicit y of Vi polysaccharide vaccine against whole-cell killed (WCK) typhoid vacc ine. Methods. The study was carried out on young adult recruits (aged 18-25 year s) of the Malaysian Air Force. A total of 125 subjects received the Vi poly ssacharide vaccine and 114 received the WCK vaccine. Findings. The Vi vaccine was significantly less reactogenic than the WCK va ccine with regard to systemic and local reactions. Following administration of the Vi vaccine, seroconversion rates (defined as the percentage of subj ects with a 4-fold rise of baseline antibody level) of 75.5% and 67% were o bserved at 2 weeks and 6 weeks, respectively, after immunization, compared with 25% and 31.3% among recipients of the WCK vaccine. Of the 110 Vi vacci nees with serological data, 21 (19%) had high, seroprotective, pre-immuniza tion levels of anti-Vi antibodies (greater than or equal to 1 mug/ml). The majority of subjects in this group came from a region in Malaysia which is known to have high typhoid endemicity. Interestingly, these antibody levels were boosted considerably following administration of vaccine at a level t hat was 5-fold higher than in subjects with low pre-immunization levels, In contrast, the seroconversion rates in those receiving the Vi vaccine were higher in subjects with low pre-immunization levels of anti-Vi antibodies ( 76-84%), compared to those with protective levels of greater than or equal to 1 mug/ml prior to immunization (48-57%). Conclusions. The study reaffirms the safety and efficacy of the Vi polysacc haride vaccine and identifies a hitherto unrecognized advantage in its use, Le. it is a potent immunogen that boosted considerably the protective anti body levels among a significant number of immunologically sensitized indivi duals living in typhoid-endemic regions.