BSE controls final report, 20 December 2000, section 4, Present and likely future size of the vCJD epidemic

12. By 1 December 2000 there were 87 definite and probable cases of vCJD of which 5 were still alive and 3 awaited confirmation⁷,⁸. There has been one confirmed case in the Republic of Ireland⁹ and two confirmed and one suspected case in France. The cases have been distributed roughly 50/50 between the sexes. The age range at death is from 14 to 74 with a mean age of 27.

13. At the present time, to predict the size and shape of the epidemic of vCJD in humans with any degree of certainty, information is needed about

• the size of the infectious dose for humans and how this varies between individuals;
• the nature and extent of the effects of the variation in susceptibility of the population to vCJD (see paragraph 14);
• the level of exposure of the public to the infectious agent and the period over which they were exposed.
• the average duration of the incubation period (time from exposure to symptoms) after exposure and infection;
• the major significant routes of exposure and transmission;
• the number at present infected but without symptoms.

14. While no data currently exist, some of these factors may vary with gender, age or ethnicity. In addition, we know that there are genetic factors which appear to govern susceptibility to the disease, since all cases so far of vCJD have had one particular type of genetic background¹⁰ (about 40% of the total population fall into this category¹¹). Furthermore, genetic variation in susceptibility is a characteristic of all human TSEs. It is possible that homozygous individuals simply represent the earliest group to show symptoms. In the case of Kuru, this variation in susceptibility affected the length of incubation but did not confer absolute resistance¹².

15. A number of assumptions need to be made to estimate the likely size of the vCJD epidemic. Given that no data currently exist with which to test the validity of those assumptions, particularly for the precise pattern of exposure of people to the infectious agent and the susceptibility of those who are not of the same genotype as the victims to date, not too much weight should be given to any one number. On present understanding, perhaps the best that can be said is that the final number of people affected might lie between a few hundred and well over a hundred thousand.

16. In summary, therefore, since there are still great uncertainties about the size of the epidemic, it is not possible to do a quantitative cost-benefit assessment for the various protective measures in place at present. Furthermore, many of the cases occurring now are likely to be due to exposures that occurred before the controls in the late 1980s and before the current, more effective post 1996 controls. The BSE epidemic is now rapidly declining, and the control measures are rigorously enforced, radically reducing the risk of exposure. In addition it should be emphasised that to calculate risk reduction of any measure on the basis solely of the cases of vCJD so far diagnosed inevitably underestimates the benefit.

17. The issue of the costs and benefits of the controls in relation to the size of the vCJD epidemic is addressed further in paragraphs 81 to 89.

Previous section | Next section

6 - Answer to Parliamentary question. House of Commons Official Report, 21 November 2000, Col 160W
7 - Department of Health Press Release, 1 December 2000
8 - These figures relate to date of death rather than date of onset of the disease since the date of onset of symptoms is very difficult to pinpoint.
9 - It is understood that the Irish case lived in England between 1990 and 1994.
10 - They are methionine homozygotes at codon 129 of the prion protein gene.
11 - Will RJ et al, A new variant of Creutzfeldt Jakob Disease in the UK, Lancet, 347 921-925 (1996)
12 - Cervenáková et al, Phenotype-genotype studies in Kuru: implications for new variant Creutzfeldt-Jakob disease. Proc Nat Acad Sci, 95, (22), 13239-13241, 1998