Since September 2017, yellow fever cases have been reported across several states in Nigeria. From 1 January to 31 August 2021, a total of 1,312 suspected cases were reported in 367 Local Government Areas (LGAs) across 36 States and the Federal Capital Territory (FCT).
A total of 45 blood samples were sent to the Institut Pasteur in Dakar (IPD) and 31 samples tested positive by Plaque reduction neutralization test (PRNT). Of these 31 PRNT positive cases, twelve cases had a history of yellow fever vaccination. Two deaths were reported among the remaining 19 non-vaccinated PRNT positive cases (Case fatality ratio: 11%). These 19 PRNT positive cases were reported from: Enugu (seven cases), Anambra (three cases), Benue (three cases), Delta (two cases), Oyo (two cases) Niger (one case) and Osun (one case) State. Investigations into the PRNT-positive cases are ongoing.
Nigeria has documented gaps in population immunity against yellow fever. According to WHO-UNICEF 2020 estimates, the national immunization coverage for yellow fever was 54% in 2020 which is below the threshold of 80% necessary to protect against outbreaks. In the nine states reporting PRNT positive cases, the routine immunization coverage declined between 2018 and 2020 and was below 80% in 2020. These states include Anambra, Benue, Delta, Enugu, Imo, Niger, Ondo, Osun and Oyo. Six states reported coverage below 50% (Anambra, Delta, Enugu, Imo, Osun, and Oyo states).
Between 2019 and 2020, preventive mass vaccination campaigns were conducted in six (all LGAs) of the nine states. Coverage was reported to be high (>90%) in Delta and Ondo States but lower (<80%) in Anambra, Benue, Niger, Osun and Oyo States. Additionally, in Enugu State, nine out of 17 LGAs organized reactive mass vaccination campaigns in 2020, while in Imo State, mass vaccination activities have not been organized in recent years.
In addition, yellow fever surveillance is sub-optimal. Not all suspected cases are documented, presumptive positive cases are not always investigated, and investigations of confirmed cases and confirmatory testing results are delayed. Additionally, vaccination status which is critical for interpretation of laboratory results and case confirmation may not be reported as part of the investigation.
Public health response
Yellow fever response activities are coordinated by Nigeria Center for Disease Control through a multi-agency Yellow Fever Technical Working Group. The current response includes the following:
Distributed yellow fever preparedness and response guidelines to all states;
Commenced yellow fever IgM testing for the South-East and South-South geopolitical zones at University of Nigeria Teaching Hospital Enugu and University of Benin Teaching Hospital Benin City, respectively;
Additional yellow fever Information, Education and Communication materials has been printed for dissemination;
Ongoing development of yellow fever training manual for the training of healthcare workers on yellow fever identification, management, documentation, and reporting;
Ongoing daily media monitoring across social and traditional media platforms to detect yellow fever related signals.
Weekly monitoring and analysis of surveillance data across the country to guide response activities;
Monitoring the effective use of Surveillance and Outbreak Response Management System (SORMAS) in all affected states to improve real-time reporting;
Supporting National Primary Health Care Development Agency in the planning meetings for yellow fever preventive mass vaccination campaign for Quarter 4 (November 2021 in Abia, Ebonyi, Imo, Taraba and selected wards and LGAs in Borno States;
Coordination of sample collection and transportation from affected states to the national testing laboratories and regular testing of yellow fever samples in all the testing laboratories. Additionally, ensuring availability of reagents and consumables in all testing laboratories;
Engaging with state epidemiologists for weekly updates is planned.
WHO risk assessment
Nigeria is a high-risk country for yellow fever and is recognized as a high priority country to the global Eliminate Yellow Fever Epidemics (EYE) Strategy. The re-emergence of yellow fever in September 2017 in Nigeria has been marked by outbreaks over a wide geographical area. A combination of vaccination and vector control strategies is the most important means of preventing infection. Yellow fever is endemic in Nigeria and, due to suboptimal immunization coverage in most states and nationwide, the risk of spread is high. Entomological investigations have confirmed the presence of the vectors, Aedes aegypti and Aedes africanus in several states across the country.
Given the low routine immunization coverage, coupled with the poor performance of reactive mass immunization activities, indicate an ongoing risk in susceptible populations and thus, a risk of serious public health impact.
To address the risk, the country has engaged in a multi-year plan to complete preventive mass vaccination campaigns targeting all eligible persons aged 9 months to 44-year-old, supported through the EYE and global partners. Since 2017, a total of 22 of 36 states including FCT have completed these campaigns.
National and State public health authorities are currently responding to several concurrent outbreaks (COVID-19 pandemic, Lassa fever, and a widespread cholera epidemic), which are straining the available limited resources, especially human resources to conduct investigations and response activities. Additionally, the recent relaxation of COVID-19 measures could increase population mobility and the potential risk for spreading yellow fever to urban areas.
Yellow fever is an acute viral haemorrhagic disease transmitted by infected mosquitoes and has the potential to spread rapidly and cause serious public health impact. The disease is preventable using a single dose of yellow fever vaccine, which provides immunity for life.
Although there is no specific treatment, supportive care to treat dehydration, respiratory failure and fever and antibiotic treatment for associated bacterial infections can reduce mortality and is recommended. Viraemic cases should stay under mosquito nets during the day to limit the risk of spread to others through bites of mosquitoes.
Routine yellow fever vaccination was introduced to Nigeria’s Expanded Programme on Immunization in 2004. However, due to sub-optimal yellow fever vaccination coverage, population immunity in most areas around the country remains below herd immunity thresholds (≥80%). The EYE Strategy was launched in Nigeria in 2018, yet its implementation at the national and sub-national level remains low. Preventive campaigns for yellow fever are being implemented in Nigeria in six phases. The target is to accelerate and complete these vaccination campaigns nationwide by 2024.
As yellow fever is endemic in Nigeria, it is a priority country for the EYE strategy. Accelerated phased vaccination campaigns are planned to cover the entire country by 2024. Vaccination is the primary intervention for prevention and control of yellow fever. In urban centres, targeted vector control measures are also helpful to interrupt transmission. WHO and partners will continue to support local authorities to implement these interventions to control the current outbreak.
WHO recommends vaccination against yellow fever for all international travellers from 9 months of age going to Nigeria. Nigeria requires a yellow fever vaccination certificate for all travellers aged 9 months or over as a condition of entry.
Yellow fever vaccines recommended by WHO are safe, highly effective and provide life-long protection against infection. In accordance with the IHR (2005), the validity of the international certificate of vaccination against yellow fever extends to the life of the person vaccinated with a WHO approved vaccine. A booster dose of approved yellow fever vaccine cannot be required of international travellers as a condition of entry.
WHO encourage its Member States to take all actions necessary to keep travellers well informed of risks and preventive measures including vaccination. Travellers should also be made aware of yellow fever symptoms and signs and instructed to seek rapid medical advice when presenting signs. Viraemic returning travellers may pose a risk for the establishment of local cycles of yellow fever transmission in areas where the competent vector is present.