A recent study of stored tissue samples, published in the Health Protection Report in August 2012, found abnormal prion protein in 16 appendices out of 32,441 samples. This suggests a prevalence of about one in 2,000, which remains statistically consistent with results from an earlier appendix survey. This estimate measures the prevalence of abnormal prion protein in appendix tissues within the population covered. We cannot know for certain whether this is a good indicator of risk in relation to potential blood-borne routes of infection, such that blood taken from donors with abnormal prion protein in appendix tissue would transmit prion infection. However, risk assessments, prepared for the independent scientific Advisory Committee on Dangerous Pathogens (ACDP), are based on the presumption that this could occur. In February 2013, ACDP agreed and published an updated variant Creutzfeldt-Jakob disease (vCJD) and blood components risk assessment, which takes into account the recent prevalence study data. A copy of this document has been placed in the Library, and is publicly available on the Department's website. The prevalence of infective blood donors remains unknown. Not all individuals in the study would be of an age eligible to donate blood, nor is it clear whether presence of abnormal prion protein in tissues such as the appendix indicates that the blood of such a donor would transmit vCJD. Precautionary measures are assessed in the context of the fundamental uncertainties about prevalence.