Salmonella enterica serovar Typhi strain Ty21a (Ty21a)

Salmonella enterica serovar Typhi strain Ty21a is a live-attenuated bacterial strain used as the active component of the oral typhoid vaccine, Vivotif. It was developed through chemical mutagenesis of the virulent Ty2 strain, resulting in multiple genetic defects, most notably a mutation in the galE gene (galactose-4-epimerase deficiency). This mutation causes the bacterium to undergo lysis upon the uptake of galactose, effectively limiting its ability to cause systemic disease while remaining immunogenic (Germanier & Fürer, 1975). When administered orally, Ty21a colonizes the gut-associated lymphoid tissue and induces a multifaceted immune response involving mucosal IgA, systemic IgG, and cell-mediated immunity (WHO, 2018). Because it is a live organism, its efficacy is significantly reduced if taken concurrently with antibiotics or certain antimalarials like proguanil, which can kill the vaccine strain before an immune response is mounted (Vivotif Package Insert). It is primarily indicated for the active immunization of adults and children against typhoid fever caused by S. Typhi in endemic regions or for travelers. While highly effective for prevention, Ty21a is not a molecular drug target but rather a biological therapeutic agent used to prime the human immune system.

Other names

VivotifSalmonella typhi Ty21aS. Typhi Ty21aLive oral typhoid vaccine Ty21aTy21a vaccine strain

Ty21a acts as a live-attenuated vaccine that mimics a natural infection with Salmonella typhi to stimulate the host's immune system. It induces the production of protective secretory IgA antibodies in the intestinal mucosa, serum IgG antibodies, and activates specific cell-mediated immune responses, including cytotoxic T lymphocytes, which provide long-term protection against wild-type S. Typhi infection (Germanier & Fürer, 1975; Levine et al., 1987).