Field Effectiveness of Vi Polysaccharide Typhoid Vaccine in the People's Republic of China

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Abstract
To the Editor—Field investigations in the People's Republic of China to determine the protective efficacy of typhoid Vi polysaccharide vaccine (Vi vaccine) were done among students attending a middle school [1]. Vi vaccine was offered to students even during an outbreak of typhoid fever. The efficacy was 73% among those immunized before the outbreak and 71% among vaccine recipients during the outbreak. Many vaccine failures during the vaccine efficacy trials might have been caused by infection with Salmonella serotype Typhi strains that were lacking the Vi antigen. Vi-negative Salmonella Typhi cause a disease indistinguishable from that caused by a Vi-positive strain [2]. On their entry to an intracellular niche, Vi-negative Salmonella Typhi are protected from the Vi antibody [3]. Since Vi vaccines would not elicit any cellular immunity, they would not stop intracellular replication of Salmonella Typhi.

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