Transmissible spongiform encephalopathies
The implications for vaccine safety of the emergence of certain transmissible spongiform encephalopathies (TSEs) have received the attention of the Committee for several years. Animal products, for example gelatin, are used as an excipient in some vaccines. Although it is clear that the major risk of human exposure to TSEs such as bovine spongiform encephalopathy (BSE) is from food sources, and that the risk from vaccines remains theoretical, WHO has compiled precautionary regulatory guidelines. These guidelines define tissue infectivity categories and outline risk management strategies not only for vaccines but also for blood products and pharmaceutical products.
GACVS noted several developments. On the positive side: (i) the annual number of deaths from variant Creutzfeld Jacob disease (vCJD) in the United Kingdom, which has the highest global burden of known vCJD, is in decline; (ii) the availability of WHO reference preparations has enabled demonstration of increasing sensitivity in laboratory diagnostic detection methods, but it is clear that more needs to be achieved; and (iii) data are available to show that the gelatin manufacturing processes reduce the infective load of abnormal misfolded prion protein associated with TSEs (PrPTSE) spiked in starting material by several log orders. On the negative side, GACVS heard that: (i) transmission of vCJD by blood transfusion is possible; (ii) susceptibility to vCJD infection is not confined to the methionine homozygous prion protein gene (PRNP) genotypes; (iii) people with heterozygous PRNP genotypes, who constitute the majority of the population, may be clinically silent carriers of vCJD; and (iv) recent data show that transmission of PrPTSE has been detected in muscle tissue from some animals, and the issue of spread of TSE through the infection of small ruminants has been raised following demonstration of PrPTSE in a goat.
Given these developments there is a need to update the relevant parts of the WHO guidelines but not, at this stage, measures pertaining to vaccines. GACVS noted that surveillance systems are crucially important in developing countries and elsewhere where BSE undoubtedly exists but is not detected and reported. The Committee will be maintaining a watching brief on the situation.