Study area/ study design
This is a cross-sectional descriptive outbreak investigation and response of yellow fever in Edo State, Nigeria as at January 2019. Edo State is one of the states in the South-South geo-political zone of the country with 18 LGAs [13]. The clusters of “cases of a strange illness” that initially affected four LGAs namely, Esan Central, Esan West, Owan East and Uhunmwode LGAs, later increased in both severity and geographic coverage extending to 12 LGAs between September 2018 and January 2019.
Advocacy visits were paid to key stakeholders to provide information about the presence and purpose of the team in the state and to obtain detailed information on the current situation and activities undergone. At the community level, advocacy visits were made to community leaders by the RRT detailing the nature and risks associated with the disease and preventive measures. Community leaders were sensitised on the case definition for yellow fever.
Operational case definition
A modified standard case definition for YF from the integrated disease surveillance and response (IDSR) technical guidelines (2013) for Nigeria was adapted as the working case definition and utilised for the purpose of identifying suspected cases of YF residing in the communities in Edo State [14].
The study population included persons who met the case definitions of yellow fever as follows:
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i.
Suspected Case: Any person residing in Edo State with acute onset of fever, with jaundice appearing within 14 days of onset of the first symptoms with or without bleeding from September 1, 2018 to January 12, 2019.
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Probable Case: A suspected case whose sample was IgM positive / PCR positive/metagenomics positive in a national laboratory in the absence of YF vaccination within 30 days of onset of illness with an epidemiological link to a confirmed case or an outbreak and positive post-mortem liver histopathology.
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Confirmed Case: A probable case and the detection of YF-specific IgM, detection of a four-fold increase in YF IgM and/or IgG antibody titres between acute and convalescent serum samples, detection of YFV-specific neutralising antibodies at WHO Regional Reference Laboratory, Institut Pasteur.
Following the establishment of case definitions for the outbreak, the activities detailed below were subsequently carried out during the outbreak investigation:
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Active case search
Active case search was done in line with the YF preparedness and response guideline and YF field investigation guide [15, 16] Active case search was conducted by the RRT at the health facilities and communities. For health facilities (HF), a retrospective review of HF records (registers/case notes) took place at the medical records, outpatient and inpatient and the laboratory sections from September 1, 2018 to January 12, 2019 was done. Patients who met the case definitions were added to a specific yellow fever outbreak line list.
Two approaches were used in the community active case search. Community leaders were sensitised on the case definition for yellow fever. The first approach was to assemble community members together in a place approved by the community leader where they were sensitised and examined for symptoms and signs of YF. The second approach was a house-to-house case search where the RRT visited every house in the community with an assigned community guard by the community leader.
Any person that met the case definitions for suspected case was added to a line-list and their blood sample collected. Detailed case investigation was carried out on all the confirmed cases. Human blood sample management.
The RRT facilitated sample management (collection, packaging, and transportation) as part of outbreak response activities. All suspected cases had 5mls of venous blood collected by the laboratory team. The samples stored in plain bottles were centrifuged at 500 g-1000 g for 5 min to obtain sera. The sera were collected into cryovial tube(s), stored at + 2 to + 8 °C or frozen at − 20 °C degrees Celsius (°C). These samples were triple packaged and shipped under good cold chain through a contracted courier company to the NCDC Central Public Health Laboratory (CPHL), Yaba, Lagos for IgM serology. Positive (presumptive positive) samples were sent to the World Health Organization (WHO) Regional Reference Laboratory, Institut Pasteur (IP) Dakar where both real-time polymerase chain reaction (RT-PCR) and plaque reduction neutralization test (PRNT) were used for final confirmation.
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Risk assessment
Risk assessment was done at the state level using a set of 14 criteria for the assessment:
Each criterium was given a maximum score of one and a minimum score zero (1 or 0): Total score was 14 while least score was 1. Earned score was divided by the total score and multiplied by 100. The percentage scores were graded thus: 70-100% is very high risk; 40-69% is moderate risk and below 40% is low risk. Data were collected using a pro forma, entered and analysed using Microsoft Excel.
In addition, a risk communication gap assessment to review existing documents and reports, inventory of existing communication materials and key informant interviews. Coordination and system strengthening, yellow fever jingle, media plan, training schedule for healthcare workers and community engagement were carried out.
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Verbal autopsy
Verbal Autopsy (VA) was used to estimate disease burden, mortality, and under-reporting of yellow fever as part of the National Yellow fever Outbreak Response Strategy. A case of VA was defined as “any death of a family member(s) who prior to death developed acute onset of fever and jaundice appearing within 14 days in a person who resided in Uhunmwode, Esan West, Esan Central and Owan West or any other LGAs within Edo State between September 1, 2019, to January 12, 2019” [7]. A questionnaire was used to collect data from family members. Any death in the community that met the case definition was included. However, all cases line listed in the VA were verified with the state surveillance data. Those already captured in the state surveillance data were excluded from the report.
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Entomological surveillance
An entomological survey was conducted in the first four LGAs to identify the presence of the yellow fever vectors. The approaches used to establish the presence of the vectors, Aedes mosquitoes, in the locations visited include (i) larval sampling, which was designed to collect immature stages (larvae and pupae) of the vectors. (ii) Ovitraps were designed to collect Aedes mosquito eggs. (iii) modified Human Landing Catch (mHLC), designed to collect adult mosquitoes Two types of adult collection traps were deployed: Biogents’-sentinel trap and CDC UV light trap [17,18,19].
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Rapid Vaccination Coverage Assessment
Rapid Vaccination Coverage Assessment (RVC) was conducted in the four LGAs where the outbreak started to determine the yellow fever vaccination status of children 10 years and below in the community, as part of the national YF outbreak response strategy. A systematic sampling of alternate houses was used to identify those to be included. The assessment began where the RRT met with the community leader and the team subsequently moved in a clockwise direction. Children below the age of one and above 10 years were excluded. A living first-born child between 1 and 10 years in each house was studied until 10 children per settlement were identified and their caregivers interviewed. A caregiver at each selected house was asked for the history of yellow fever vaccination as well as documentary evidence in the routine immunisation (RI) cards to show that the child had YF vaccination. Sighting of the immunisation card and date of yellow fever vaccination was evident that the child received YF vaccination.
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vi.
Yellow fever reactive mass vaccination campaign
A request for YF reactive mass vaccination campaign was made through the International Coordinating Group (ICG) for vaccine provision. Upon approval by the ICG, pre-implementation and implementation microplans were developed. The campaign strategy was a fixed and temporary fixed post campaign strategy targeting the age groups of 9 months to 44 years (85% of total population).
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Data management and Analysis
Yellow fever specific investigation data tools were used for different activities, and these include.
Active case search: the yellow fever specific line-list in Excel template was used and analysed with Microsoft Excel software.
Verbal autopsy: data was collected using a structured-interviewer-administered questionnaire. Data was entered and analysed using Epi-Info software.
Risk assessment: Checklist was used for data collection and analyses with Microsoft Excel software.
Entomology: A customized excel template was used in collection of entomology data.
Rapid yellow fever vaccination coverage assessment: a checklist was used to collect data. Data was entered and analysed using Epi-Info software. RMVC data were collated and analysed using the yellow fever mass vaccination campaign database in Microsoft Excel.
All data analysis done were descriptive data analysis using percentages, proportions, and frequencies.