Background: West Nile virus (WNV) is endemic in the United States and transmissible by transfusion. Since 2003, the US blood supply has been screened by nucleic-acid tests (NAT) for WNV in minipools (MP-NAT) of 6 or 16 specimens. WNV infection begins with low-level viremia detectable only by individual testing (ID-NAT) and no detectable WNV antibodies. Viremia then increases to levels detectable by MP-NAT, and antibodies become detectable; later, viremia decays to levels detectable only by ID-NAT before becoming undetectable. All but 1 documented WNV transmission by transfusion involved blood components negative for WNV antibodies, raising the question whether WNV antibody-positive blood components with low levels of WNV RNA are infectious.
Methods: Specimens from 102 viremic donors with and without WNV antibodies were used to investigate infectivity in cultures of Vero cells and human monocyte-derived macrophages (MDMs).
Results: In Vero cell culture, 54 (74%) of 73 WNV antibody-negative specimens and 10 (36%) of 28 WNV antibody-positive specimens were infectious. In a random subset of 20 specimens tested in MDM culture, 7 (88%) of 8 WNV antibody-positive specimens and 12 (100%) of 12 WNV antibody-negative specimens were infectious.
Conclusion: WNV antibodies do not always protect susceptible cells from WNV infection in vitro. RNA positivity in the presence of antibody cannot be ignored as a theoretical risk for blood recipients and needs further investigation.