Typhoid fever vaccines
Typhoid fever is caused by a Gram-negative bacterium, Salmonella typhi. A similar but often less severe disease is caused by Salmonella serotype paratyphi A. The clinical spectrum of typhoid fever can vary from mild illness with low-grade fever, malaise and dry cough to severe abdominal discomfort with multiple complications that can become fatal.
Two typhoid vaccines are currently available:
- A polysaccharide vaccine based on the purified Vi-antigen, known as Vi-PS vaccine. This single-dose intramuscular or subcutaneous injectable Vi vaccine provides about 70% protection against blood culture-confirmed typhoid fever for at least 3 years.
- A live attenuated oral vaccine known as Ty21a, made with attenuated S. typhi strain Ty2 and is available in capsules. A liquid formulation also used to be available but now discontinued due to lack of demand for it. Extensive trials in children in Chile, Egypt and Indonesia showed protective efficacy of 33-67% against blood-culture confirmed typhoid fever for the capsule Ty21a, and a fourth dose of the vaccine increased the protective efficacy of the vaccine significantly. In two trials in school-aged children in Chile, the capsule Ty21a vaccine had a 67% protective efficacy after three years and 62% protective after seven years.
The WHO Strategic Advisory Group of Experts on immunization (SAGE) recommends that, in view of the continued high burden of typhoid fever and increasing antibiotic resistance, and given the safety, efficacy, feasibility and affordability of the above two licensed vaccines, countries should consider their programmatic use to control endemic disease.