Outbreak at a glance
During the period 12 January to 15 March 2022, a total of 53 suspected yellow fever cases, including six deaths, have been reported from Isiolo county, central Kenya. Two samples tested positive by RT-PCR and six were positive by ELISA, indicating probable yellow fever cases. The last reported yellow fever outbreak in Kenya was in 2011. WHO assesses the public health risk as high at the national and regional levels.
On 4 March 2022, the Ministry of Health (MoH) of Kenya declared an outbreak of yellow fever in the county of Isiolo in central Kenya (around 270 km north of the capital Nairobi). As of 15 March, a total of 53 suspected yellow fever cases have been reported from Isiolo county during the period 12 January to 15 March 2022, including six deaths (case fatality ratio: 11.3%) (Figure 1). The majority of the cases are males (47 cases; 88.7%), and the average age of cases is approximately 28 years (range: 3-78 years).
Figure 1. Epidemiological curve of yellow fever cases by date of symptoms onset and outcome, Isiolo county, Kenya, 1 January – 15 March 2022 (n=53).
Suspected cases presented with symptoms of fever, jaundice, and muscle and joint pain. Overall, eleven wards within Isiolo county have been affected, with the highest number of cases reported in: Chari (21 cases; 39.6%), Cherab (14 cases; 26.4%) and Garba Tulla (5 cases; 9.4%) (Figure 2).
Figure 2. Number of yellow fever cases by ward, reported from 12 January to 15 March 2022 in Isiolo county, Kenya (n=53).
As of 15 March, samples were collected from 34 suspected cases (64%), and were tested for yellow fever at the national laboratory – Kenya Medical Research Institute – through reverse transcriptase-polymerase chain reaction (RT-PCR) and IgM antibodies by the enzyme-linked immunosorbent assay (ELISA). Two samples (6%) were found to be positive by RT-PCR, and six (18%) were positive for IgM antibodies by ELISA. On 8 March 2022, the samples were shipped to the yellow fever regional reference laboratory – Uganda Virus Research Institute (UVRI) – for confirmatory testing. At the point of writing this report, confirmation is still pending, and there is uncertainty around current laboratory results due to the presence of malaria positivities amongst the samples tested (n=5; 15%).
There is no information on the vaccination status of the reported cases, however, Isiolo and the surrounding counties have no history of yellow fever vaccination campaigns. Kenya has not conducted large-scale preventive mass vaccination campaigns, and yellow fever vaccination is included into the national routine immunization schedule (i.e. for children at 9 months) only in four counties in the northwest of the country (not directly bordering Isiolo) deemed to be at highest risk. According to WHO-UNICEF, at the national level, the overall estimated coverage through routine immunization is 7% of the target population. This is far below the recommended 80% population coverage to provide herd immunity against outbreaks.
Epidemiology of Yellow Fever
Yellow fever is an epidemic-prone mosquito-borne vaccine preventable disease caused by an arbovirus transmitted to humans by the bites of infected Aedes and Haemagogus mosquitoes.
Forty-seven countries in Africa (34) and Central and South America (13) are either endemic for, or have regions that are endemic for, yellow fever. Since September 2021, nine countries in the WHO African Region (Cameroon, Chad, Central African Republic, Côte d’Ivoire, the Democratic Republic of Congo, Ghana, Niger, Nigeria, and Republic of Congo) have reported human laboratory confirmed cases of yellow fever in areas that are at high risk for the disease and have a history of yellow fever transmission and outbreaks. These outbreaks are occurring in a large geographic area in the West and Central regions of Africa. These reports signal a resurgence and intensified transmission of the yellow fever virus. The outbreaks have included areas that have previously conducted large-scale mass vaccination campaigns but with persistent and growing gaps in immunity due to lack of sustained population immunity through routine immunization and/or secondary to population movements (newcomers without history of vaccination).
The Government has implemented a national incident management structure to manage the outbreak and has developed a response plan, deploying a rapid response team to Isiolo and neighbouring counties to determine the extent of the outbreak, identify the at-risk population, conduct a risk assessment, initiate risk communication and community engagement activities and implement integrated vector control measures.
The Government and the WHO, together with partners (UNICEF, Food and Agriculture Organization, Amref Health Africa, Kenya Red Cross, Living Goods, Médecins Sans Frontières, US Centers for Disease Control and Prevention, World Vision, Action Aid) mobilized resources to support response activities, including a proposed request to the International Coordinating Group for vaccine provision for reactive yellow fever vaccination in Isiolo with possible extension to any other counties found to have cases or have imminent risk.
Kenya is endemic for yellow fever and is classified as a high-risk country in the Eliminate Yellow Fever Epidemics (EYE) Strategy. Previous outbreaks have been reported in 1992, 1993, 1995 and 2011 in the western part of the country (Rift Valley zone). In 2016, two imported cases from Angola were also reported. Epidemic spread of yellow fever is a risk in Kenya as the estimated routine yellow fever vaccination coverage is very low among the target population (7%) and is limited in scope to four counties in the western part of the country (Baringo, Elgeyo Marakwet, West Pokot and Turkana).
Yellow fever has never been reported in Isiolo county, which is a pastoralist and remote area, around 270 km north of the capital Nairobi. The total population of Isiolo county was estimated at 268 002 in 2019 according to the Kenya National Bureau of Statistics. Although Isiolo is in the central region of Kenya and does not share international borders, it is marked by frequent population movements. Garba Tulla ward shares borders with South Wajir and Western Garissa counties that have been experiencing massive pastoralists movements, amplified by current drought conditions. There is also massive refugee migration from neighbouring Somalia into Garissa county, Kenya. A national park is also located in the area near Isiolo, and the presence of informal mining activities attracting large numbers of workers as well as the presence of non-human primates has been noted, underlying the potential risk of spread to other areas.
Considering the above-described scenario, the risk is assessed as high at the national and regional levels, and low at the global level.
WHO continues to monitor the epidemiological situation and review the risk assessment based on the latest available information.
Surveillance: WHO recommends close monitoring of the situation with active cross-border coordination and information sharing, due to the possibility of cases in neighbouring countries and the risk of onward spread. Enhanced surveillance with investigation and laboratory testing of suspect cases is recommended.
Vaccination: Vaccination is the primary means for prevention and control of yellow fever. Review of the risk analysis and scope of immunization activities to protect the population could help avert the risk of future outbreaks.
Vector control: In urban centres, targeted vector control measures are also helpful to interrupt transmission. As a general precaution, WHO recommends avoidance of mosquito bites including the use of repellents and insecticide treated mosquito nets. The highest risk for transmission of yellow fever virus is during the day and early evening.
Risk communication: WHO encourages its Member States to take all actions necessary to keep travellers well informed of risks and preventive measures including vaccination. Travellers should be made aware of yellow fever symptoms and signs and instructed to rapidly seek medical advice if presenting signs and symptoms suggestive of yellow fever infection. Viraemic returning travellers may pose a risk for the establishment of local cycles of yellow fever transmission in areas where a competent vector is present.
International travel and trade: WHO advises against the application of any travel or trade restrictions on Kenya. Yellow fever vaccination is required by national authorities for international travellers over one year of age entering Kenya, and it is recommended by WHO for travellers aged 9 months or over, except for those whose itineraries are limited to the following areas: the entire North Eastern Province, the States of Kilifi, Kwale, Lamu, Malindi and Tanariver in Coastal Province, and the cities of Nairobi and Mombasa.
In accordance with the IHR (2005) third edition, the international certificate of vaccination against yellow fever becomes valid 10 days after vaccination and the validity extends throughout the life of the person vaccinated. A single dose of WHO approved yellow fever vaccine is sufficient to confer sustained immunity and life-long protection against yellow fever disease. A booster dose of the vaccine is not needed and is not required of international travellers as a condition of entry.
We would like to acknowledge the National Authorities of Kenya for the information provided to enable the publication of this Disease Outbreak News.
Citable reference: World Health Organization (25 March 2022). Disease Outbreak News; Yellow Fever – Kenya. Available at: https://www.who.int/emergencies/disease-outbreak-news/item/yellow-fever-kenya
Outbreak at a glance