Vi capsular polysaccharide vaccine
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The Vi capsular polysaccharide vaccine (or ViCPS) is one of two vaccines recommended by the World Health Organisation for the prevention of typhoid (the other is Ty21a). It was first licensed in the US in 1994 and is made from the purified Vi capsular polysaccharide from the Ty2 Salmonella Typhi strain; it is a subunit vaccine. A newer conjugate form of the vaccine (Vi bound to a non-toxic recombinant Pseudomonas aeruginosa exotoxin, or Vi-rEPA) has enhanced efficacy, including protection of children under 5 years of age.
The vaccine may be used in endemic areas in order to prevent typhoid. It is also commonly used to protect people who are traveling to parts of the world where typhoid is endemic.
The vaccine is injected either under the skin or into a muscle at least seven days before traveling to the typhoid-affected area (the CDC recommend 14 days). The vaccine is not effective in children under the age of two; children under the age of two would normally have all their food regulated by their parents and should therefore be expected to be at low risk of exposure to typhoid.
To maintain immunity, the vaccine should be repeated every three years.
Efficacy and duration of protection
The vaccine offers effective protection the first year after being given (with between 50% to 80% efficacy) and the second year (31% to 76%), but not after that.
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