In 1988, a programme of leprosy chemoprophylaxis, employing a supervised, single 25 mg/kg dose of rifampicin, was implemented in the Southern Marquesas Islands. Of the 2786 inhabitants, 275 1 (98·7%) were treated. In addition, 3144 South Marquesans living elsewhere in French Polynesia were administered the same chemoprophylaxis. During the following 10 years, seven leprosy patients were detected among those who had been administered chemoprophylaxis. Of these, two were very likely missed cases

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... f leprosy, and cannot be considered a failure of chemoprophylaxis. The epidemiometric proj ection model, based on cases of leprosy observed in the Southern Marquesas during the 20 years preceding implementation of the programme, predicted that 17 leprosy cases could be expected in the South Marquesan population if no chemoprophylaxis were given. In fact, only five cases were detected in the treated population, a number significantly smaller than 17, suggesting that the chemoprophylaxis was 70% effective, assuming that no change of detection rate would have occurred without chemoprophylaxis. However, during the 10 years following implementation of the chemoprophylaxis programme, the detec tion rate in the Polynesian population that was not administered chemoprophylaxis declined by about 50%. Therefore, the effectiveness of the chemoprophylaxis was only 35-40%.