Control of Typhoid Fever in Bangkok, Thailand, by Annual Immunization of Schoolchildren with Parenteral Typhoid Vaccine

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Abstract
The number of cases of typhoid fever in Bangkok, Thailand, began to increase sharply in 1974 and peaked in 1976. In 1977, as part of a national typhoid immunization program, Thai schoolchildren aged seven to 12 years began to receive annually a single 0.25-ml subcutaneous dose (2.5 × 108 organisms) of a heat/phenol-inactivated typhoid vaccine. Isolations of Salmonella typhi in Bangkok decreased from 880 (4.6% of all blood cultures) in 1976 to 54 (0.3% of all blood cultures) in 1985. The case ratio of S. typhi to Salmonella paratyphi A infection declined from 4.1:1 before the epidemic (1970–1973) to 0.9:1 after the epidemic (1984–1985), and the proportion of cases of typhoid fever occurring among children aged seven to 12 years significantly decreased from 30% to 10%. During the same periods S. paratyphi A isolation rates did not significantly decrease (in terms of either total number or percentage of cases) in school-aged children. Thus, mass vaccination of schoolchildren in Thailand with the heat-inactivated typhoid vaccine has been closely associated with a sharp decline in typhoid fever in Bangkok during an epidemic and with continuous control after the epidemic.

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