Epidemiological determinants of the pattern and magnitude of the vCJD epidemic in Great Britain

Understanding the epidemiology and aetiology of new-variant Creutzfeldt-Jak ob (vCJD) disease in humans has become increasingly important given the sci entific evidence linking it to bovine spongiform encephalopathy (BSE) in ca ttle and hence the wide exposure of the population of Great Britain (GB) to potentially infectious tissue. The recent analysis undertaken to determine the risk to the population from dorsal route ganglia illustrated the dange r in presenting point estimates rather than ranges of scenarios in the face of uncertainty. We present a mathematical template that relates the past p attern of the BSE epidemic in cattle to the future course of any vCJD epide mic in humans, and use extensive scenario analysis to explore the wide rang e of possible outcomes given the uncertainty in epidemiological determinant s. We demonstrate that the average number of humans infected by one infecti ous bovine and the incubation period distribution are the two epidemiologic al factors that have the greatest impact on epidemic size and duration. Usi ng the time-series of the BSE epidemic and the cases seen to date, we show that the minimum length of the incubation period is approximately nine year s, and that at least 20% of the cases diagnosed to date were exposed prior to 1986. We also demonstrate that the current age distribution of vCJD case s can only arise if younger people were either exposed to a greater extent, more susceptible to infection, or have shorter incubation periods. Extensi ve scenario analyses show that given the information currently available, t he very high degree of uncertainty in the future size of the epidemic will remain for the next 3-5 years. Furthermore, we demonstrate that this uncert ainty is unlikely to be reduced by mass screening for late-stage infection.