This 7/02 report, among other things,  evaluated the potential impact of the new vCJD donor restrictions on the U.S. blood supply.  The report provides history and background on the donor restrictions and good info in Appendix II, "Risk of vCJD Infection through Blood  Transfusion Is Unknown"

TSE Related Snippets:

The nation's blood supply can compensate for donors lost because of new
donor restrictions designed to further reduce the risk of vCJD
transmission. The increased incidence of BSE in the cattle herds in
continental Europe has prompted FDA, the Red Cross, and DOD to
implement more stringent donor deferral policies. Initial FDA guidance
published in 2000 recommended the exclusion of individuals who had
spent 6 months or more in the United Kingdom. This guidance was
tightened in 2002 to exclude individuals who had spent 3 months or more
in the United Kingdom and individuals who have spent a cumulative total
of 5 years in European countries where there is a risk of acquiring
vCJD by eating contaminated meat. FDA estimates that its new deferral
policy will further reduce the risk of possible exposure to vCJD by 23
percent but will disqualify about an additional 5 percent of donors in
the United States. .

In response to the possibility that vCJD could be transmitted through
blood transfusions, in November 1999, FDA recommended deferring by
April 2000 blood collections from individuals who had resided or
traveled in the United Kingdom for a total of 6 months or more from
1980 through 1996. [Footnote 22] In recognition of the evolving BSE
epidemic, FDA issued a more restrictive policy in January 2002.

In August 1999, FDA issued guidance that recommended prohibiting
donations from individuals who had resided or traveled in the United
Kingdom for a total of 6 months or more from 1980 through 1996, a
period during which that country experienced an epidemic of BSE in
cattle.[Footnote 31] In response to the detection of BSE in cattle in
European herds, in January 2002 FDA issued guidance to expand this
recommended exclusion to prohibit donations from individuals who had
spent a cumulative 3 months in the United Kingdom from 1980 through
1996, or 5 years or more in a European country since 1980. The portion
of FDA's new guidance pertaining to residents of the United Kingdom and
France took effect on May 31, 2002, and the deferral of donors who have
resided in other European countries will take effect on October 31,
2002. FDA's guidance exempts donors of source plasma who had resided in
Europe for 5 years from 1980 through 1996, but it prohibits source
plasma donations from those who had resided in the United Kingdom for
at least 3 months from 1980 through 1996. The guidance also recommends
indefinite deferral of source plasma donors who have spent 5 or more
years cumulatively in France from 1980 to present.[Footnote 32]
:
Because so little is known about the etiology of vCJD, estimates of the
public health benefits from blood donor exclusions related to vCJD are
uncertain. It has not been established that vCJD is transmissible
through blood, and no tests to diagnose vCJD or detect vCJD in blood
have been developed. Nonetheless, laboratory experiments point to a
theoretical risk of transmission of vCJD through blood. (See app. II
for a description of scientific research on vCJD.):

Transmission of vCJD by human blood or plasma has not been
demonstrated, and no laboratory or epidemiological studies have shown
that blood from donors infected with vCJD carries the disease. For
example, at least 20 people in the United Kingdom have received blood
or blood components from donors who later developed vCJD. Although
relatively little time has passed, none of the recipients of the blood
have developed vCJD. Studies of patients with vCJD and a prior history
of receiving blood transfusions have not revealed any cases of vCJD
among the donors involved.
Nonetheless, laboratory experiments point to a theoretical risk of
transmission of vCJD through blood. For example, tissue samples from
vCJD patients have found the agents that cause vCJD, protein molecules
known as prions, in human lymph tissue, such as the tonsils and the
spleen. Since white blood cells known as B lymphocytes also circulate
through these tissues and are potentially involved in the pathology of
vCJD, researchers suggest that these circulating lymphocytes may carry
infectivity in blood.[Footnote 37] Experiments with animals have shown
that blood infected with vCJD-like agents contain low-levels of
infectivity. In addition, one group of researchers has recently
demonstrated that BSE can be experimentally transmitted between sheep
by blood transfusion.[Footnote 38] However, results from this
experiment may not be representative of the human manifestation of
vCJD.

Epidemiological Predictions:

Researchers are limited in the conclusions they can make concerning
vCJD and blood safety, and in predicting the future number of vCJD
cases. Important variables in determining the probability of BSE
transmission to humans, such as route of exposure, genetic
susceptibility, and dose, remain unproven. Further, the incubation
period for vCJD is unknown but is probably many years. Citing the
current modest number of additional deaths in the United Kingdom caused
by vCJD (there were 28 confirmed or probable vCJD deaths in the United
Kingdom in 2000 and 20 in 2001), some researchers suggest that the
epidemic will not reach the hundreds of thousands once thought
possible. As a result, the projected number of total cases has been
revised downward to just a few hundred or few thousand cases, with
fewer than 100 new cases occurring per year.[Footnote 39] Such revised
estimates are based on varying assumptions regarding the average
incubation period and when individuals were infected.[Footnote 40]

The ambiguity of the scientific evidence regarding vCJD transmission
through blood is reflected in the divided vote of FDA's advisory
committee (the Transmissible Spongiform Encephalopathies Advisory
Committee, or TSEAC) in favor of the expanded donor deferral. The
committee voted 10 to 7 in June 2001 to move forward with the proposed
changes, but several members expressed concern about the expanded
deferral's impact on blood availability, the effectiveness of current
efforts to control human exposure to BSE in the United Kingdom, and the
reliability of European surveillance data.

Detection Tests for vCJD under Development:

The scientific uncertainties surrounding vCJD would be greatly reduced
if a diagnostic test existed to confirm the presence or absence of vCJD
in human blood. While tests are being developed, it could be some time
before an accurate test will be available to screen blood for the vCJD
agent. Tests do exist to detect vCJD prions in some human tissues, such
as brain tissue, tonsils, and appendixes, but no suitable tests are
available to detect vCJD infections in blood. Prions are different than
viral and bacterial pathogens, which contain nucleic acids. Some
pathogens and viruses trigger the human body to release specific
antibodies, which may be detected in the blood. For example, both HIV
and hepatitis elicit antibodies in the blood that can be detected in a
blood test. At this point, most scientists believe that prions, such as
those involved in vCJD, do not contain nucleic acids and do not elicit
the production of antibodies. This poses a challenge in designing a
blood test, which must be 100,000 times as sensitive as assays that
already exist for detecting prions in tissues. If a test were approved,
it would be required to be extremely sensitive to minimize the
possibility of false positives, which would unnecessarily defer from
donating blood many individuals who did not actually have the vCJD
agent in their blood.

[21] See U.S. General Accounting Office, Mad Cow Disease: Improvements
in the Animal Feed Ban and Other Regulatory Areas Would Strengthen U.S.
Prevention Efforts, GAO-02-183 (Washington, D.C.: Jan. 25, 2002).

[22] See U.S. General Accounting Office, Blood Supply: Availability of
Blood to Meet the Nation's Requirements, GAO/HEHS-99-187R (Washington,
D.C.: Sept. 20, 1999).

[31] FDA guidance documents are not regulations, and they do not have
the force of law. In practice, however, all blood banks have treated
the guidance as requirements and have implemented donor restrictions
that are at least as restrictive as those recommended by FDA.

[32] FDA exempted source plasma from donors who had spent time in
Europe during the period indicated because plasma-derivative processing
can remove the agent thought to cause vCJD and because of concerns
about maintaining sufficient supplies of important plasma-derivative
therapies (see app. I). Plasma derived from whole blood, however, is
subject to the same restrictions as whole blood.

[33] The model FDA used to estimate donor exposure to the BSE/vCJD
agent assumes a linear risk related to the duration and likelihood of
dietary exposure to beef from BSE-affected cattle. Compared with the
United Kingdom, other European countries have experienced few vCJD
cases and a lower incidence of indigenous BSE in cattle herds. For
these reasons, FDA assigned a lower risk estimate to time spent in
other countries. For example, FDA estimates that the risk in European
nations is 1.5 percent to 5 percent of the risk in the United Kingdom.
If the risk of exposure to BSE in Europe is 5 percent of the risk in
the United Kingdom--at the high end of FDA's estimates--a pan-European
deferral of 5 years (60 months) would be equivalent to the new 3-month
deferral for cumulative travel or residence in the United Kingdom. See
Food and Drug Administration, A Guidance for Industry: Revised
Preventive Measures to Reduce the Possible Risk of Transmission of
Creutzfeldt-Jakob Disease (CJD) and Variant Creutzfeldt-Jakob Disease
(vCJD) by Blood and Blood Products (Rockville, Md.: Jan. 2002).

[37] A.F. Hill and others, "Investigation of Variant Creutzfeldt-Jakob
Disease and Other Human Prion Diseases with Tonsil Biopsy Samples,"
Lancet, vol. 353, no. 9148 (1999), pp. 183-189.

[38] F. Houston and others, "Transmission of BSE By Blood Transfusion
in Sheep," Lancet, vol. 356, no. 9234 (2000), pp. 999-1000.

[39] J.N. d'Aignaux and others, "Predictability of the UK variant
Creutzfeldt-Jakob Disease Epidemic," Science, vol. 5547, no. 294
(2001), pp. 729-31.

[40] P. Brown, "Bovine Spongiform Encephalopathy and Variant
Creutzfeldt-Jakob Disease," British Medical Journal vol. 322, No. 7290
( 2001), pp. 841-44, and Rebecca Love, "Has the Variant Creutzfeldt-
Jakob Disease Epidemic Hit Its Peak?" Lancet, vol. 358, no. 9291
(2001), p. 1432.