Molecular and Serological Evidence of Oropouche Virus Circulation in Asymptomatic Blood Donors During the 2023–2024 Outbreak in Manaus, Brazil

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BACKGROUND: Oropouche virus (OROV) re-emerged in Brazil in 2023, causing over 26 000 confirmed cases through December 2025. A reassortant lineage was linked to neurologic complications and congenital infections due to vertical transmission. Although transfusion transmission has not been reported, asymptomatic viremia in blood donors could represent an unrecognized route of infection. METHODS: We conducted nucleic acid testing (NAT), genomic, and serological surveillance among blood donors in Manaus, Brazil, during the 2023-2024 OROV outbreak. Minipools (MPs) of 18 donations from November 2023 to May 2024 were tested using a validated RT-qPCR assay. A subset of NAT-reactive pools underwent whole genome sequencing and phylogenetic analysis. Serosurveys in November 2023 and June 2024 assessed changes in population immunity. The RNA-detectable window was estimated by integrating NAT yield with seroincidence. RESULTS: Among 661 MPs, representing 11 898 donations, 43 (6.5%) were OROV RNA-reactive, peaking in January 2024. About half contained viral RNA concentrations above the limit of quantification. Donor-derived viral genomes clustered with the reassortant lineage circulating in Brazil. Donor seropositivity increased from 13.0% to 29.7%, corresponding to approximately 390 000 estimated infections in Manaus, substantially higher than reported case numbers. The estimated MP RNA-detectable window was 5.4 days (95% CI: 3.8-7.0). CONCLUSIONS: OROV RNA detection in donor samples, together with high-titer viremia, phylogenetic identity to the outbreak strain, and postoutbreak seroconversion, raises questions about blood safety. Continued donor and recipient surveillance will be vital to determine transfusion-transmission risk and inform transfusion safety policies.

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