The 1970 yellow fever epidemic in Okwoga District Benue Plateau State, Nigeria. 2. Immunity survey to determine geographic limits and origins of the epidemic.

Serological surveys undertaken to define the geographic limits of the 1970 rural yellow fever epidemic in Okwoga District, Nigeria, indicated that surrounding areas of Benue Plateau State and East Central State were not involved. However, the surveys uncovered a separate focus of unrecognized, recent epidemic yellow fever in Mbawsi, in southern East Central State. The highest proportions of yellow-fever-immune sera outside the Okwoga and Mbawsi foci were found in zones of Guinea savannah in the Benue River basin.