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BACKGROUND : Human West Nile virus (WNV) infection has been documented in the eastern United States since its discovery there in 1999. Epidemics of WNV encephalitis and meningitis raise concern that transmission of WNV may occur through voluntary blood donations.

STUDY DESIGN AND METHODS : Case onset dates from the 1999 Queens, NY, epidemic of WNV encephalitis and meningitis, and historic data on viremia in humans are used to estimate the number of cases that were viremic throughout the epidemic. Estimates of the inapparent-to-apparent WNV infection ratio, the proportion of asymptomatic infections reported in a seroepidemiologic survey coincident with the epidemic, and the population size are used to infer the WNV transfusion-transmission risk. Statistical resampling methods are used.

RESULTS : The maximum and mean risk of WNV transmission (/10,000) from donors in Queens were estimated as 2.7 (95% CI, 0.9-5.6) and 1.8 (95% CI, 1.4-2.2), respectively. The risk peaked in late August, with very low risk before August and after September.

CONCLUSION : Although most WNV-infected individuals have subclinical infections, these data suggest a low prevalence of viremia throughout the Queens epidemic and subsequent low risk of transmission of WNV by blood transfusion.