A one health investigation of the 2024 yellow fever outbreak in Bayelsa State, Nigeria: Epidemiology, response and lessons learned

BACKGROUND: Yellow fever, a mosquito-borne viral disease, is transmitted to humans through bites from infected mosquitoes. Bayelsa State, in Southern Nigeria, reported an outbreak of yellow fever in 2024 despite immunization efforts. This study outlines the outbreak findings and lessons learned. METHOD: A secondary analysis of case-based surveillance data obtained from IDSR OO1C was conducted from January to December 2024, focusing on demographics such as age, sex, location, and time. Immunization coverage and entomological findings were also assessed. FINDINGS: From January to December 2024, 85 suspected yellow fever cases were reported, 13 (15.29%) of which were presumptively positive and 6(7.06%) confirmed using RT-PCR and PRNT. Among suspected cases, 43(50.59) were males with an average age of 19.21yrs and SD of ± 16.33. One death was reported amongst confirm cases, giving a case fatality rate (CFR) of 16.7%. Among the six confirmed cases, one case each were identified in four LGAs of the state (Yenagoa, Southern Ijaw, Nembe and Brass), while two cases were reported in one LGA (Kolokuma/Opokuma). The January 2023 mass immunization campaign achieved 87% coverage. Entomological surveys in 5 communities, including areas with confirmed cases, identified three Aedes species (Aedes albopictus, Ae. luteocephalus, and Ae. simpsoni complex). Larval indices indicated a high epidemic risk across surveyed areas. CONCLUSION: The findings show that while surveillance and vaccination remain important, robust environmental and vector control measures are essential for effective yellow fever prevention.