The response to re-emergence of yellow fever in Nigeria, 2017.

Yellow fever (YF) is an acute viral hemorrhagic disease caused by the YF virus (arbovirus) which continues to cause severe morbidity and mortality in Africa. A case of YF was confirmed in Nigeria on the 12th of September 2017, 21 years after the last confirmed case. The patient belongs to a nomadic population with a history of low YF vaccination uptake, in the Ifelodun Local Government Area (LGA) of Kwara State, Nigeria. An active case search in Ifelodun and its five contiguous LGAs led to the listing of 55 additional suspect cases of YF within the period of the outbreak investigation between September 18 to October 6, 2017. The median age of cases was 15 years, and 54.4% were males. Of these, blood samples were collected from 30 cases; nine tested positive in laboratories in Nigeria and six were confirmed positive for YF by the WHO reference laboratory in the region; Institut Pasteur, Dakar. A rapid YF vaccination coverage assessment was carried out, resulting in a coverage of 46% in the LGAs, with 25% of cases able to produce their vaccination cards. All stages of the yellow fever vector, Aedes mosquito were identified in the area, with high larval indices (House and Breteau) observed. In response to the outbreak, YF surveillance was intensified across all States in Nigeria, as well as reactive vaccination and social mobilisation campaigns carried out in the affected LGAs in Kwara State. A state-wide YF preventive campaign was also initiated.