89th Meeting, April 26-27, 2007

Gaithersburg, Maryland


Topic III: Issues Related to Implementation of Blood Donor Screening for Infection with West Nile Virus

I.  Issue: FDA seeks comments from the Committee on issues related to implementation of donor screening with a licensed nucleic acid test (NAT) for West Nile virus (WNV) RNA.

II. Background

Infection with WNV

WNV is an arthropod-borne virus that belongs to the Japanese encephalitis complex of flaviviridae.  WNV is a small (50nm), spherical, lipid enveloped virus enclosing a single-stranded positive sense ribonucleic acid (RNA) genome.

WNV is primarily transmitted in birds through mosquito bites, while humans are incidental hosts.  Incidental mosquito borne infection may also occur in other animals including horses, cats, squirrels, and domestic animals.  WNV outbreaks have been reported in Europe, the Middle East, and Russia during the past decade and have been associated with human encephalitis and meningitis.  A poliomyelitis-like illness of acute asymmetrical flaccid paralysis in the absence of pain or sensory loss has also been reported more recently. 

The pre-clinical incubation period is estimated to range from 2 – 14 days following infection by mosquito bite.  Most people infected with WNV (~80%) do not develop any illness. However, approximately 20% of people who become infected with WNV will develop mild symptoms.  These symptoms are often indistinguishable from other viral infections, and may include fever, headache, body aches, nausea, vomiting, eye pain, occasionally with a skin rash on the trunk of the body, and swollen lymph glands.

CDC has previously estimated that approximately 1 in 150 persons infected with WNV develop a more severe form of the disease with neurological involvement. More recent publication using better defined data estimates that 1 in 256 to 353 persons infected with WNV will develop neurological symptoms (Busch et al. EID 2006; 12:395).  Severe disease may culminate in fatal encephalitis in about 1 in 1000 infections.  The risk of severe disease increases by age, with persons older than 50 years at particular risk.  Persons who are immunocompromised appear to be at very high risk. Severe illness may include encephalitis, meningitis, meningoencephalitis, or acute flaccid paralysis, which may occur singly or in combination.  Symptoms may include: headache, high fever, neck stiffness, stupor, disorientation, coma, tremors, convulsions, and muscle weakness or paralysis.  Severe symptoms may for last weeks to months, and some permanent neurologic impairment may occur.  Case fatalities among patients who were hospitalized in the United States with severe WNV illness have ranged from 10 to 14%.   

West Nile virus was first detected in New York City in the summer of 1999, and later progressed to a nationwide epidemic.  Since 2002, large annual outbreaks of WNV have begun in the late spring and summer with extension into the late fall.  Cumulative outbreaks of WNV have resulted in over 23,000 human cases of neurological disease and at least 931 fatalities reported to the CDC.  Based on the number of cases of neurological illness it is estimated that between 1.7 and 2.6 million people have been infected with WNV since 1999.  In 2002, CDC investigators documented other modes of WNV transmission including by organ transplantation, blood transfusion, and feeding of breast milk.

Donor Screening for WNV in the USA

In the wake of the massive outbreak of WNV in 2002, cooperative efforts among various DHHS agencies, state public health laboratories, blood establishments and test kit manufacturers resulted in the rapid development of two investigational NAT systems for blood donor screening and their implementation in June 2003.  NAT screening for WNV was seen as a potential measure to prevent viral spread by transfusion of blood and components and from donation of organs and tissues.

Initial screening protocols included NAT performed on minipools of samples from six or 16 donations, depending on the test kit manufacturer.  Individual donation tests are performed on all samples comprising a reactive minipool to identify and retrieve the infected unit(s) from the blood supply.  Screening of blood donations for WNV is performed all year round using MP-NAT.  As a result of nationwide implementation of WNV testing under IND, over 2,000 presumptively viremic donations were removed from the blood supply since 2003, preventing between two to six thousand transmissions of WNV to recipients of blood components from these units.

On December 1, 2005 FDA licensed the first qualitative in vitro nucleic acid assay system, the Procleix® WNV Assay, developed by Gen-Probe Inc. and marketed by Chiron Corporation, for the detection of WNV RNA in the plasma specimens from individual human donors including volunteer donors of whole blood and blood components and other living donors. It is also intended for use in testing plasma specimens to screen organ donors when the specimen is obtained while the donor’s heart is still beating , and in testing blood specimens to screen cadaveric (non-heart beating) donors.  Other WNV NAT assays for donor screening continue to be used for donor screening under the approved INDs.

On March 2, 2007 FDA approved the Procleix® WNV Assay on the Procleix® TIGRIS system, which is the first fully automated NAT system for the detection of WNV RNA in human plasma manufactured by Gen-Probe Inc, and marketed by Chiron Corporation. This system is approved for the detection of WNV RNA in the plasma specimens from individual human donors including volunteer donors of whole blood and blood components and other living donors. It is also intended for use in testing plasma specimens to screen organ donors when the specimen is obtained while the donor’s heart is still beating, and in testing blood specimens to screen cadaveric (non-heart beating) donors.  This system has been licensed for testing either individual donations or minipools of 16 donations.  Full automation enhances compliance with current Good Manufacturing Practice (cGMP) by reducing the potential for human error while accelerating blood screening and enhancing blood and tissue safety.

Use of Criteria to Trigger ID-NAT

Despite implementation of donor screening for WNV using MP-NAT in 2003, there were six cases of transmission of WNV by transfusion.  Retrospective studies performed in 2003 revealed that only 75% of WNV infected units were detected by MP-NAT test.  Based on the results of these studies the blood organizations voluntarily implemented ID-NAT in specific regions with high WNV activity during the epidemic in 2004. Trigger criteria to switch to ID-NAT were to take into consideration both the frequency of positive MP-NAT that contain individually reactive donations and the absolute number of WNV reactive donations identified from MP-NAT. 

The restricted use of ID-NAT is due to the limited availability of resources to perform ID-NAT using semi-automated assay platforms.  The implementation of ID-NAT is initiated when a threshold determined by each center is reached.  This threshold is most often based on the number of MP-NAT-positive screening test results obtained in a one-week rolling interval, or a cumulative rate for positive screening results.  The criteria used were 1 in 1000 donations reactive for WNV and 2 MP-NAT positives in a week.  Based on voluntary industry criteria, a facility would resume MP NAT when a minimum of 7 days has passed without a WNV NAT-reactive donation that repeated as reactive by the same or alternate NAT.  There were no reported/documented transfusion related transmissions of WNV in 2005 using these criteria.

In 2006 there were two breakthrough cases of WNV transmission resulting from a nonreactive MP-NAT donation from which red blood cells (RBC) and fresh frozen plasma (FFP) components were transfused to two immunosuppressed recipients who had neurological WNV infection and no other apparent source of WNV exposure.  The donor involved in the 2006 transmission cases was from a rural area of South Dakota, where substantial WNV activity in birds, mosquitoes and humans occurred during the 2006 epidemic.  This required reexamining the trigger criteria for optimal sensitivity of WNV testing.  In particular, a need has been recognized to improve communication amongst blood collection centers that would permit rapid identification of geographic areas with increased incidence of WNV in donors.

III. Issues for Implementation of Blood Donor Screening for WNV

A.  Donor screening, additional testing, and donor counseling

We believe, based on the scientific data, that the implementation of licensed NAT for WNV RNA for screening volunteer donors of Whole Blood and blood components for transfusion has improved the safety of the nation’s blood supply and should be recommended.

According to current testing algorithm, MP-NAT reactive results are followed by pool resolution to identify the unit(s) that led to the pool reactivity. Current data show that the sensitivity of repeat NAT in index donation specimens (i.e. same or independent specimen from the index donation) using either the same screening assay or an equally sensitive alternate NAT along with antibody testing has a positive predictive value of 98%. Therefore, FDA considers it appropriate that specimens (including index specimens) from donations reactive by ID-NAT should be retested using the same assay, or with an alternate NAT with sensitivity comparable to that of the screening assay.  We also encourage the use of antibody tests on the ID-NAT reactive units identified by MP-NAT resolution since antibody testing contributes to the positive predictive value of the retest.

Current data show that about 10% of the initially reactive (IR) specimens that were true positive based on subsequent donor seroconversion did not have repeat reactive results either on the same or alternate NAT upon further testing.  Therefore, FDA considers that IR specimens should not be regarded as False Positive based solely on negative test results obtained on further testing using the screening assay or alternate NAT, and/or for antibodies to WNV.

Based on these scientific data, FDA considers it appropriate that donations that are ID-NAT reactive should not be released for transfusion, and whether these donors should be deferred from donation for a period of 120 days.  For donor counseling purposes, a donor can be considered True Positive on the index donation when the index specimen is repeat reactive by NAT or tests positive for antibodies to WNV, even if repeat NAT is non-reactive.  FDA considers it appropriate that donors with IR results that have negative results on further testing should be informed about a possible infection with WNV.  FDA encourages blood collection establishments to invite such donors for additional counseling and follow-up testing at least 30 days after the IR donation.  The follow up specimen would be tested by both ID-NAT and an antibody test.

B.  Triggering Criteria for ID-NAT

Although a fully automated NAT system has been licensed, there is a lack of sufficient data for FDA to recommend uniform criteria for triggering ID-NAT with optimal sensitivity that meets blood center needs.  For these reasons, at the present time uniform criteria for triggering ID-NAT for WNV testing may not be appropriate.   Pending development of suitable uniform criteria, FDA considers it appropriate that blood establishments should follow the triggering criteria proposed by AABB for implementation of ID-NAT, and that each blood center follow an established SOP for this decision process. These criteria, which will be described in greater detail at the meeting, include triggering based on the weekly number or frequency of IR specimens identified on MP-NAT resolution within a predefined cohort of donors defined geographically (e.g. by zip code).  Additionally, AABB is developing a web-based tool to improve timeliness of geographical reporting of WNV-infected blood donors to facilitate and expedite focal ID-NAT triggering decisions.   FDA encourages continued data collection and assessment of various triggering criteria that would support any future recommendations.

C. Issues Addressed in Previous Guidance

FDA has not changed its thinking regarding issues addressed in a guidance document published in June 2005 (see

1.  Unit Management

In regard to unit management, FDA’s current consideration is that all units that are WNV NAT reactive should be discarded, independent of any results of additional testing.  All in-date blood and blood components, including recovered plasma from a donor who is presumptively viremic (WNV NAT-reactive), should be retrieved and quarantined. 

2.  Recipient Notification

Blood establishments receiving information regarding WNV reactive unit(s) may consider tracing records and notifying transfusion services regarding “relevant units” from prior collectionsTransfusion services may then consider notifying treating physicians of prior recipients.

3.  Product Labeling

The container label and instruction circular that reflects the results of WNV NAT should be consistent with the labeling for other infectious disease markers upon implementation of licensed NAT.  WNV reactive unit should not be shipped or used except as provided in FDA approved programs and/or research or for autologous use only and such units should be labeled with appropriate warnings.