The attenuated Ty21a strain is a mutant of Salmonella typhi, which is deficient in the enzyme UDP-4-galactose epimerase. This results in the organisms being unable to effectively metabolize galactose. When grown in the presence of adequate amounts of galactose, the organism accumulates galactose-containing metabolites and ultimately undergoes spontaneous lysis. In the presence of a restricted supply of galactose the organism develops the smooth lipopolysaccharide coat believed to be necessary for immune response. In the intestine, where galactose is normally present, it is however unable to survive for long. The vaccine strain cannot be detected in the stools after 3 days following oral ingestion.
In one clinical study conducted in Egypt, in children above 6 years of age, oral ingestion of the vaccine as a solution preceded by a dose of sodium bicarbonate to reduce gastric activity (in order to reduce lysis of the organism in the stomach), provided approximately 95% protection against typhoid. In another study, conducted in Chile, enteric coated capsules provided approximately 70% protection. The duration of protection conferred by Vivotif Oral (typhoid vaccine) remains to be fully established. However, repeat vaccination is not considered necessary within 12 months after initial vaccination.
Last reviewed on RxList: 9/30/2008
This monograph has been modified to include the generic and brand name in many instances.
Additional Vivotif Oral Information
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