We conducted a retrospective descriptive study of the epidemiology of the yellow fever outbreak in Nigeria over 2 years spanning September 2017-Septemeber 2019.
Nigeria is located in the West African on the Gulf of Guinea between Benin and Cameroun with an estimated population in 2019 of 198 million persons as projected from the 2006 Census. Nigeria covers an area of 923,768km2 with savannah and forest vegetation in the northern and southern parts with climatic conditions which promote spread of arthropods. Forest reserves make up about 10% of the total land area in Nigeria, with a reported population of primates (18).
Nigeria is regarded as one of the high-risk countries for yellow fever transmission and a high priority for implementation of the eliminating yellow fever epidemics (EYE) strategy. In September 2018, a yellow fever case was confirmed in a 12-year-old girl in Ifelodun Local Government Area (LGA) of Kwara state. This outbreak spread across multiple states from September 2017 to October 2019. Eleven reactive vaccination campaigns covering 65 LGAs were conducted from September 2017 to October 2019. Also, the national laboratory network was expanded from four to seven laboratories for preliminary testing by serology. Other interventions included the activation of the incident management system (IMS) and functional public health emergency operation centres (PHEOCs) were also in place to support the outbreak response
We reviewed the country programme data with a specific focus on the immunisation and surveillance and immunisation country database reported by all 36 states plus the Federal Capital Territory (FCT) Abuja and the 774 LGAs maintained by the Government of Nigeria and supported by the World Health Organisation at national levels.
We measured and compared the number of suspected and confirmed cases of yellow fever reported by LGA and state levels throughout the review as well as the number of Interventions conducted in response to these outbreaks. We also tracked the trends of these outbreaks over time to describe the epidemiological patterns. Yellow fever cases were also summarised by age and sex distribution as well as case fatality rates patterns across states between September 2017-2019. Fever surveillance is case-based surveillance in Nigeria and information on all suspected cased is documented and collated at LGA, State and National levels. Based on the national guidelines, suspected cases which fit the standard case definition are investigated with blood samples collected for laboratory confirmation as displayed in Figure 1. In Nigeria, there are four designated laboratories for yellow fever testing within the WHO network and the National Reference Laboratory in Abuja. Samples are preliminarily tested via serology and IgM positive cases considered as probable cases. Samples for all probable cases are subsequently shipped to the Regional Reference Laboratory located at the Institute Pasteur, Dakar Senegal where confirmation is done via repeat serology and subsequent real-time polymerase chain reaction (PCR-rt) and plaque reduction neutralisation tests (PRNT). Results from RRL Institute Pasteur, Dakar are shared with Nigeria routinely after the tests are conducted.
All confirmatory results received in the country at national levels are officially communicated immediately to the states. The state teams subsequently conduct, an outbreak investigation with documentation of clinical features, vaccination status, travel history and other variables. Entomological surveys are also documented to assess the presence of the vector and the risk of amplification and urban spread.
An outbreak report is subsequently developed and based on the recommendations, an Interagency Coordination Group (ICG) request completed to support reactive vaccination campaigns to control the spread.
All vaccination campaign data are collated, and post-campaign surveys conducted to validate the quality of the campaign.
All programme data were collated through a Microsoft Access® database collected at Zonal and National levels and updated weekly. Immunisation data was also collected through the same system and cleaned following regular data harmonisation conducted by the Data Management Team within WHO and the Nigeria Centre for Disease Control (NCDC). Immunisation data are managed by the National Primary Health Care Development Agency (NPHCDA) and supported by WHO. Entomological reports, detailed case investigations forms and Laboratory data were also reviewed and summarised for variables of interests.
Non-available data variables or lost data was accounted for, and assumptions for entries/replacements stated accordingly
All available data were analysed using Microsoft Access and Excel and presented as frequencies, proportions and trends.
No ethical standards were bridged during the process of this review. All data used were made access from the available yellow fever surveillance and immunisation date