Post-Mass Vaccination Epidemiology of Suspected and LaboratoryConfirmed Yellow Fever in Rivers State, Nigeria (2020-2025)

Abstract

Background: Yellow Fever (YF) is a re-emerging, vaccine-preventable arboviral disease of major public health importance in Nigeria. Rivers State conducted a preventive mass vaccination campaign in early 2020. Sustained surveillance is essential to detect outbreaks and identify residual immunity gaps in the post-campaign period. Objective: To describe the epidemiology of suspected and confirmed Yellow Fever cases in Rivers State from January 2020 to August 2025. Methods: A retrospective descriptive analysis of state-integrated disease surveillance and laboratory data was conducted. Suspected cases were defined using national guidelines. Laboratory confirmation was based on Yellow Fever IgM ELISA at a regional reference laboratory, with confirmatory plaque reduction neutralization testing (PRNT) at a WHO reference laboratory. Demographic, geographic, and vaccination characteristics were analyzed. Results: No suspected cases were reported in 2020. From August 2021 to August 2025, 181 suspected cases were recorded, of which 9 (5.0%) were laboratoryconfirmed. Confirmed cases occurred in late 2021 and mid-2025. Males accounted for 88.9% of confirmed cases, and 44.4% were infants under one year. None of the confirmed cases had documented YF vaccination. Confirmed infections occurred in only 5 of 23 LGAs, indicating focal transmission. Conclusion: Rivers State experienced sporadic, geographically focal Yellow Fever activity following the 2020 mass vaccination campaign. Confirmed infections occurred exclusively among unvaccinated individuals, highlighting both vaccine effectiveness and persistent immunity gaps. Strengthening routine immunization, targeted supplementary campaigns, and timely laboratory feedback remain essential to prevent future outbreaks.