It is now clear that there is a risk from the transmission of vCJD through blood transfusion. The reason for this is not completely clear in that attempts to demonstrate infectivity in blood by inoculation into the brains of animals has show no transfer. However the sheer quantity of blood involved in a blood transfusion from a human must mean that the risk is much greater than can be seen with any animal methods.
For a long period it was unclear if CJD was transferred through blood transfusion or not. Initially attempts were made by comparing the difference between transfusion prevalence between those that developed CJD and those that were controls. Unfortunately the number of CJD cases needed to tell the difference would have needed to be several thousands and hence this study was unhelpful.
Later much greater numbers of cases were compared using international studies and with sCJD there did not indeed seem to be any increased likelyhood of having received a transfusion in the CJD cases. Meantime it was clear that infectivity was present in the blood of scrapie cases in various species (mice, hamsters etc) and in 2003 it was shown that if sheep were bled, before they were symptomatic with scrapie, then their blood would transfer the disease to other sheep by transfusion.
From this it was no longer surprising when a patient that developed vCJD (with exactly the same symptoms as a usual patient with the disease) was found to have received the blood donation from another case of the disease before that person had vCJD symptoms.
It must now be assumed that vCJD is present in blood and that it is infective in transfusions. How to remove it or test for it is now unclear but technologies are being developed currently for this purpose. In March 2004 in UK all blood donation by someone that has already received blood as a transfusion may not be accepted.