Risk Assessment for Yellow Fever in Western Province of Zambia

IntroductionYellow fever (YF) is estimated to affect about 200,000 people annually in the tropics of Africa and South America (1) and 30,000 deaths occur worldwide (2). The majority of persons infected with YF suffer from a self-limiting febrile illness. In persons who develop symptoms, symptoms occur 3-6 days after being infected and include fever, chills, severe headache, back pain, general body aches, nausea, vomiting, fatigue and weakness to severe liver disease with bleeding. Most persons who develop symptoms improve after the initial presentation of symptoms. About 15% of cases progress to a more severe form of the disease that is characterized by high fever, jaundice, bleeding, and eventually shock and failure of multiple organs (3). There is no specific treatment for yellow fever; care is based on symptoms. Steps to prevent yellow fever virus infection include using insect repellent, wearing protective clothing, and getting vaccinated (3).Transmission of YF in Africa has been maintained by a high density of sylvatic and urban vector mosquitoes (4), mainly Aedes aegypti and Aedes africanus (5). Prevention of YF depends on avoiding mosquito bites by using repellent and wearing protective (long-sleeves, long pants and socks when outdoors) clothing and persons aged 9 months or older who live in YF endemic areas or travelling to such areas may be vaccinated to prevent infection (3).In Africa, Yellow fever was mainly a problem of the sub-Saharan countries of West Africa, but reached as far east as central Sudan and East Africa (6-10). By 2005, Yellow fever was endemic in Angola that borders Western province of the Zambia (4). Zambia as a whole country was classified as a no risk area but Western and North-western provinces of Zambia were classified as low risk areas for yellow fever. The history behind the current classification of Western province as a low risk area for yellow fever is such that before 2005, it was classified as endemic area and in the period 2005-2010 it was removed from the list of areas with risk of transmission of yellow fever virus. The decision to classify Western province as a low potential for yellow fever exposure (11) was based on a suspected case of yellow fever that was described in North-western province of Zambia in 1943 (12), 18% seroprevalence of neutralizing antibodies to Yellow fever (13) and neighbouring areas in Angola and Democratic Republic of the Congo being at risk of yellow fever risk (11). There was no recent evidence that could have been used to classify the area. Thus, a yellow fever risk assessment survey was conducted in 2013 to provide evidence for or against the classification of the area as low risk area for yellow fever.MethodsWestern province borders with Angola and had seven districts divided into 1902 Standard Enumeration Areas (SEAs). The population stood at 881,524 with a population density of 7.0 (14). Crop and livestock production as well as fishing were the main economic activities.Study population, sample size and samplingThis assessment was carried out among individuals aged nine months or older. The sample size calculation was based on the assumption that the seroprevalence for yellow fever was 7% based on the study conducted by Robinson (12). In estimating the sample size for persons aged 5 years or older, the following parameters were considered: a prevalence of 7%, desired precision or confidence interval (d) of +3%, and a design effect (DE) of 2 and an 80% response rate. Considering sex, we aimed to recruit 700 male and 700 female participants in each province. Assuming an average of 4 persons aged 5 years or older in each household, a total of 12 households in each of the 30 clusters was to be recruited in the survey. The total number of persons that would be recruited from each province was 1806, totaling 3612 from both provinces.The seroprevalence of children was about half that for older children, and in estimating the sample size for persons aged below 5 years, the following parameters were considered: a prevalence of 3. …