Estimated risk of transmission of the West Nile virus through blood transfusion in the US, 2002

BACKGROUND: The West Nile virus (WNV) epidemic in 2002 in the US saw over 3300 reported human cases of WNV disease, with over 2300 reported cases of WNV encephalitis and meningitis. The first documented cases of transfusion transmission of WNV through voluntary blood donation also occurred. STUDY DESIGN AND METHODS: Case onset dates from the 2002 WNV epidemic in the US were used to estimate the risk of transfusion-associated transmission with statistical resampling. An easily computed approximating formula for the mean risk was derived. Estimates were computed for six high-incidence states and metropolitan areas. RESULTS: Mean and maximum risk of transfusion-associated WNV transmission (per 10,000 donations) during the epidemic period for the selected states ranged from 2.12 to 4.76 and from 4.34 to 10.46, respectively; for the selected metropolitan areas, they ranged from 1.46 to 12.33 and from 3.02 to 21.32, respectively. CONCLUSIONS: Estimates of the mean risk of WNV transmission by transfusion ranged from 1.46 to 12.33 per 10,000 donations for six high-incidence metropolitan areas during the 2002 epidemic. Because the risk was highly geographically and temporally variable, computation of geographically localized estimates is recommended. The derived approximating formula for the mean risk performed well for the estimates given.