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Ty21a is a live attenuated bacterial vaccine that protects against typhoid. First licensed in Europe in 1983 and in the USA in 1989, it is an orally administered, live-attenuated Ty2 strain of S. Typhi in which multiple genes, including the genes responsible for the production of Vi, have been mutated chemically so as to render it harmless but nevertheless immunogenic. It is one of two typhoid vaccines currently recommended by the World Health Organization (the other being the Vi capsular polysaccharide vaccine).
The vaccine is given by mouth. The vaccine is presented either as enteric coated capsules or a liquid suspension. The vaccine must be stored at 2 to 8 °C, but will retain its potency for 14 days at 25°C.
Efficacy and duration of protection
The vaccine offers a statistically significant protection for the first seven years, with between 62% to 77% effectiveness at year seven.  The vaccine is most commonly used to protect travelers to endemic countries, but some agencies claim that the vaccine could be used in large scale public prevention programs.
The recommended dose varies according to country and preparation.
In the US and Canada, an initial course of 4 doses on alternate days is recommended. Full protection is achieved 7 days after the last dose. In the US, a booster dose is recommended after 5 years. In Canada, a booster dose is recommended after 7 years.
In Australia and Europe, an initial course of 3 doses on alternate days is recommended. Protection is achieved 7 days after the last dose. A booster is recommended every 3 years for people living in endemic areas, but every year for people traveling from non-endemic to endemic areas.
- Vivotif (manufactured by Crucell)
- World Health Organization (2008). "Typhoid vaccines: WHO position paper" (PDF). Weekly Epidemiological Record 83 (6): 49–60.
- Fraser A, Goldberg E, Acosta CJ, Paul M, Leibovici L (2007). Fraser, Abigail, ed. "Vaccines for preventing typhoid fever". Cochrane Database Syst Rev (3): CD001261. doi:10.1002/14651858.CD001261.pub2. PMID 17636661.
- Salisbury, D; Ramsay, M; Noakes, K. ""Chapter 33: Typhoid"". Immunisation against infectious disease (PDF). Department of Health. p. 418. ISBN 0-11-322528-8.