Update: West Nile virus screening of blood donations and transfusion-associated transmission--United States, 2003.

In 2002, transfusion-associated transmission (TAT) of West Nile virus (WNV) infection acquired through blood transfusion marked the emergence of a new threat to the U.S. blood supply. Although mosquito-borne transmission remains the predominant mode of WNV transmission, identification of TAT underscored the need for WNV screening of donated blood. In June 2003, blood-collection agencies (BCAs) implemented investigational WNV nucleic acid-amplification tests (NATs) to screen all blood donations and identify potentially infectious donations for quarantine and retrieval. This screening was performed on approximately 6 million units during June-December 2003, resulting in the removal of at least 818 viremic blood donations from the blood supply. This report summarizes the results of blood-donation screening tests conducted during 2003 and describes six cases of WNV TAT that occurred because of transfusion of components containing low levels of virus not detected by the testing algorithm. These data indicate that blood screening for WNV has improved blood safety. However, a small risk of WNV transfusion-associated transmission remains. To address this risk, changes to screening strategies are planned for 2004.