Vi capsular polysaccharide (Vi antigen) (Vi antigen)

The Vi capsular polysaccharide is a critical virulence factor and surface antigen found on Salmonella enterica serovar Typhi, the causative agent of typhoid fever (WHO, 2018). Chemically, it is a linear homopolymer of alpha-1,4-linked N-acetylgalactosaminuronic acid that is variably O-acetylated at the C-3 position (NCBI, PMID: 25135633). This capsule plays a vital role in pathogenesis by masking underlying surface structures like lipopolysaccharide, thereby protecting the bacterium from complement-mediated killing and phagocytosis by host immune cells (PubMed, PMID: 28254444). It also helps the pathogen evade the host's early inflammatory response, facilitating systemic spread. Because of its essential role in virulence and its accessibility on the bacterial surface, the Vi polysaccharide is the primary target for several typhoid vaccines, including purified polysaccharide vaccines (e.g., Typhim Vi) and Vi-conjugate vaccines (e.g., Typbar-TCV) (CDC, 2023). These vaccines work by inducing protective IgG antibodies that facilitate the clearance of the bacteria. TCVs have significantly improved the duration of protection and are now recommended by global health organizations for routine use in endemic areas. Monitoring anti-Vi antibody levels serves as a key biomarker for assessing vaccine efficacy and population immunity in clinical trials and public health surveillance.

Other names

Vi polysaccharideVirulence antigenSalmonella Typhi capsular polysaccharidePoly-N-acetylgalactosaminuronic acidVi capsular antigen

Induction of serum IgG antibodies against the Vi polysaccharide, which promote opsonophagocytosis and complement-mediated lysis of Salmonella Typhi (WHO, 2018; PubMed, PMID: 25135633).