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Typhoid vaccines are vaccines that prevent typhoid fever. Several types are widely available: typhoid conjugate vaccine (TCV), Ty21a and Vi capsular polysaccharide vaccine (ViPS). They are about 30 to 70% effective in the first two years, depending on the specific vaccine in question. The Vi-rEPA vaccine has been shown to be efficacious in children. The World Health Organization (WHO) recommends vaccinating all children in areas where the disease is common. Otherwise they recommend vaccinating those at high risk. Vaccination campaigns can also be used to control outbreaks of disease. Depending on the vaccine, additional doses are recommended every three to seven years. In the United States the vaccine is only recommended in those at high risk such as travelers to areas of the world where the disease is common. The vaccines available as of 2018 are very safe. Minor side effects may occur at the site of injection. The injectable vaccine is safe in people with HIV/AIDS and the oral vaccine can be used as long as symptoms are not present. While it has not been studied during pregnancy, the non-live vaccines are believed to be safe while the live vaccine is not recommended. The first typhoid vaccines were developed in 1896 by Almroth Edward Wright, Richard Pfeiffer, and Wilhelm Kolle. Due to side-effects newer formulations are recommended as of 2018. It is on the World Health Organization's List of Essential Medicines.