Controlled field trial on the effectiveness of one and two doses of acetone-inactivated and dried typhoid vaccine.

A controlled field trial was carried out with acetone-dried vaccine on about 35 000 people on the Tongan islands of tongatapu and 'Eua where typhoid fever is endemic. Volunteers were distributed at random into 3 groups, 1 of which received 1 dose of typhoid vaccine and a second 2 doses; the third, a control group, received tetanus toxoid. The population was vaccinated in 1966 and was studied until the end of 1973. Evaluation of the effectiveness of the vaccine was based on cases of typhoid fever that were confirmed by positive blood cultures. Morbidity rates in the 3 groups indicated that the acetone-dried vaccine used was effective and that a single dose of vaccine gave reasonable protection for a short period, but that 2 doses gave greater and longer protection. The results of this trial are compared with those of earlier field and laboratory studies.