Seroprevalence of West Nile virus among blood donors in mainland France, 2021 to 2022

BACKGROUND West Nile virus (WNV) is an orthoflavivirus maintained in an enzootic cycle between birds and ornithophilic Culex mosquitoes. Human infections are usually asymptomatic or paucisymptomatic, while severe neurological disease occurs mainly in children, elderly and immunocompromised individuals. Despite evidence of WNV circulation in France since the 1960s, nationwide seroprevalence data are unavailable. AIM We wanted to determine the seroprevalence of WNV among blood donors in mainland France to characterise the epidemiology of WNV and assess risk factors and potential implications for transfusion safety and public health. METHODS Sera from 44,490 volunteer blood donors collected in 2021–2022 in France were screened for anti-WNV IgG by ELISA on pools of up to four samples, with samples from non-negative pools tested individually. ELISAs for various flaviviruses and virus neutralisation tests were used to assess non-negative results. RESULTS Seroprevalence was low overall, similar in pools and individual samples (0.87% and 0.97%, respectively). Prevalence was 1.13% in Nouvelle-Aquitaine and 1.81% in Ile-de-France, preceding the first local human cases reported in 2023 and 2025 in these regions, respectively. The risk factors identified as being associated with WNV seropositivity were geographic (living in the south of France: Occitanie, Provence-Alpes-Côte d'Azur and Corsica) and related to ABO blood group. CONCLUSION Seroprevalence of West Nile virus in France is low but variable, suggesting that WNV may have circulated undetected in some areas. Monitoring flavivirus prevalence in blood donors can serve as an early warning system for human infections and provide valuable data for public health preparedness.