Control of typhoid fever in Bangkok, Thailand, by annual immunization of schoolchildren with parenteral typhoid vaccine.

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 x 10(8) 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.