Once the resistance sub-study has begun, the first odd-numbered village which notifies a case each day in each of the two arms will be included in the sub-study. An exhaustive list of households will be prepared in these villages, and 20 households will be randomly selected from this list. One member of each household will be randomly selected and will be invited to participate in the resistance sub-study. Only participants who know in advance that they will be absent during follow-up visits will be excluded. If this is the case, a second member of the household will be randomly selected as a replacement.