For tropical countries the World Health Organisation recommends a single dose of measles vaccine, administered at a minimum age of 9 months. In some African nations, however, up to 26% of all reported measles occurs before the age of 9 months, and many African nations have been reluctant to follow the WHO recommendation. In 1974 the Ministry of Health of the United Republic of Cameroon made several changes in the existing measles control strategy, including increasing the minimum age for measles vaccination from 6 to 9 months. Surveillance of measles in Yaounde, the capital city, during the five years after the increase in age at vaccination did not reveal a need to return to the minimum age of 6 months. In fact, by 1979, with measles vaccination coverage among children 12-23 months of age at 40%, there had been a 44% decrease in reported measles among children of all ages, including a 64% decrease in the measles attack rate among children under the age of 9 months. These observations support the one dose, 9 month minimum age measles vaccination policy in tropical Africa.