The Resurgence of Yellow Fever in Oyo State, Nigeria: What’s the Missing Link?
Authors: Bassey Bassey Enya , Braka Fiona , Onyibe Rosemary , Kolude Olufunmilola Olawumi , Oluwadare Marcus , Oluwabukola Alawale , Omotunde Ogunlaja , Iyande Oluwatobi Adeoluwa , Tella Adedamola Ayodeji , Olanike Olayiwola Suliat 
Published: June 2021

Abstract
Background:Yellow Fever is an acute viral hemorrhagic disease endemic in tropical Africa and Latin America and transmitted through infected mosquitoes. The renewed onset of yellow fever outbreaks in Nigeria followed a global trend of reports and from other African countries marking the emergence of yellow fever as a brand-new re-emerging global threat in 2016 with giant outbreaks of yellow fever reports in Angola and the Democratic Republic of Congo. Nigeria has been battling successive yellow fever resurgence since September 2017. Yellow Fever disease has no cure, but control is through vaccination and vector control. Eliminating Yellow fever Epidemic (EYE) strategy to improve high risk countries’ prevention, preparedness, detection, management and response to yellow fever outbreaks was developed by WHO in 2017 and launched in Nigeria in April 2018. Yet, poor vaccination coverage continues to be a cause for concern.Materials and Methods:We conducted a retrospective cross-sectional study that examines the resurgence of Yellow fever cases and outbreaks from 2013-2020 in Oyo State, Nigeria. The Yellow Fever data for both surveillance and immunization were the focus of the review. Surveillance data were retrieved from the State’s database reported by all 33 LGAs, maintained by the State and supported by the World Health Organization at the Zonal and State levels. The immunization data were retrieved from District Health Information Software (DHIS_2). The proportion of LGAs reporting at least one case of suspected yellow fever with blood specimen, and the number of suspected cases reported for each year within the period under review was measured. We also assessed the trend of confirmed cases, and the incidence per 100,000 population. Also, suspected cases of yellow fever were categorized into four age groups and their vaccination status assessed. The State’s annual administrative vaccination coverage for yellow fever vaccine was compared with the number of confirmed cases for each year.Results:The proportion of LGAs reporting at least a case of suspected yellow fever, with blood sample collected, ranged from 6.1% to 84.9% between 2014 and 2020 while a total of 9 confirmed (8 cases) and probable (1 case) cases of yellow fever were recorded. However, there were no confirmed cases from the year 2013 to 2016, including 2018 but upward trend of incidence of the disease per 100,000 population from 0% in 2013 through 2018, to 3.5% in 2019 and then to 5.6% in 2020 was observed. 93 of 240 (39%) suspected yellow fever cases reported during the given period were observed to have received yellow fever vaccine Conclusion: In conclusion, the resurgence of yellow fever cases in the State reiterate the State being high risk for yellow fever transmission and underline the need for viable interventions such as environmental hygiene to rid the environment of the disease vector’s ecological niche and improving vaccination coverage to provide population immunity.