Integrated Epidemiological and Entomologic Investigations of the 2022 Yellow Fever Outbreak in the Upper Eastern and Northeastern Regions of Kenya

Yellow fever (YF) remains a significant public health threat globally. Despite the absence of confirmed outbreaks since the 1992-1993 outbreak, Kenya is classified as a medium-to-high risk country. In January 2022, clusters of acute febrile illness and unexplained deaths were reported in Isiolo County. An integrated epidemiological and entomological investigation was conducted to confirm transmission, characterize risk factors, determine the geographic extent, and assess the risk of spread to the neighboring northeastern counties. Samples from suspected YF cases were tested using molecular and serological methods following WHO guidelines. Concurrent entomological surveys assessed vector species, densities, and breeding habitats to estimate transmission risk and guide vector control. N=194 serum samples (Isiolo and Garissa, n=166 [85.6%]; Wajir, Marsabit and Meru, n=20 [14.4%]) from suspected cases were tested. Serological testing identified 11 presumptive YFV IgM-positive cases (Isiolo, n=7; Garissa, n=1; Meru=2, Nairobi=1). Three of the positive cases from Isiolo (n=2), and Garissa (n=1) were confirmed by PRNT. Entomological surveys identified Aedes aegypti, Ae. furcifer, and Ae. simpsoni, all recognized YFV vectors. This investigation demonstrated active YFV transmission and elevated risk in the affected areas; highlighting the significance of integrated surveillance, vector control, and vaccination efforts to prevent future outbreaks.