This paper, including the emerging findings of the appendix prevalence study, was considered at the meeting of the Advisory Committee on Dangerous Pathogens Transmissible Spongiform Encephalopathy Risk Assessment Sub-Group on 14 July 2011.
The main conclusions reached by this independent expert group were as follows:
Early findings from an ongoing survey of appendix tissues being conducted by the Health Protection Agency (HPA) confirm the previous estimates for the prevalence of prion infection within the population, and extend this finding to older age cohorts than those examined previously.
- Evidence now suggests a lower estimate for the level of infectivity in blood. In the scenarios previously used, for example, a unit of red cells sourced from an infective donor prior to leucodepletion (removal of white blood cells, introduced in the UK in 1999) would have contained a large number (perhaps thousands) of “Infective Doses”. The evidence now available suggests that a unit contains of the order of one “Infective Dose”. The risk of transmission from an individual donor to recipient could remain substantial, but would not occur in every case.
- It is now appropriate to calibrate transmission models against observed clinical case numbers, subject to taking a precautionary approach in estimating how many vCJD infections would have shown up as clinical cases, as well as how many known cases might have been due to blood-borne infection
The Department will keep the findings of the prevalence study under close review, and will ask the CJD Incidents Panel, and other independent scientific expert committees with responsibilities for CJD related risk management decisions, to review past actions and use the lower estimate of blood infectivity during future considerations.