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November 1, 2001

A Reassessment of the Possible Extent of the UK vCJD Epidemic

 (SafetyAlerts) -  The total number of cases of variant Creutzfeldt-Jakob disease [abbreviated as vCJD or CJD (new var.)in ProMED-mail] [presumed to be] caused by eating BSE-infected beef could be far fewer than previously thought, says a new study. A research team from the London School of Hygiene and Tropical Medicine (LSHTM) say their estimates show that the total numbers likely to die from vCJD may be no more than a few thousand, rather than the tens of thousands currently predicted. Predicting how vCJD will develop is difficult because scientists lack the information needed to predict the pattern of the disease precisely. So far just over 100 people in the UK are thought to have died from the variant form of the disease.
> But the best guess until now by researchers at Imperial College has been that as many as 100 000 could fall victim to the disease. However, new research, published in the journal Science by a rival group, estimates the eventual number of deaths likely to number only a few thousand at most. Researchers at the London School of Hygiene have used a statistical method known as back calculation, which looks at how a disease is developing and then projects forward. They point out that the number of cases of vCJD has risen very slowly year on year, and that the total number of deaths is still relatively low. Members of the team believe that this pattern can best be explained if the incubation period, the time it takes for the symptoms of the disease to show, is very long. In that case those who may be infected are likely to die of other causes before symptoms show. They suggest that an alternative explanation for the disease pattern is that relatively few people ate BSE- infected meat.
> The Imperial team stand by their much higher estimates, saying that their rivals have underestimated the amount of BSE-infected meat the UK population was exposed to. Both groups agree that their predictions are far from certain. Both say there should be no relaxation in policy in protecting against the risk of cross-contamination through some surgical procedures and blood transfusions.
> Professor Peter Smith, Chairman of SEAC (the Spongiform Encephalopathy Advisory Committee), said: "When the disease was first recognized there were fears there may be tens or hundreds of thousands of cases, or even more. As the epidemic has gone on there have not been the numbers of cases that might have occurred, and looking at their pattern so far, we think it is unlikely that the eventual size of the epidemic will be more than a few thousand. We cannot rule completely rule out larger epidemics, but they seem to us at this stage to be unlikely."
> However, Professor John Collinge, head of the National Prion Clinic at St. Mary's Hospital in London, said there were flaws in the new study. He said there was likely to be a series of peaks in the number of cases of the disease over an extended period of time. [The LSHTM analysis assumes a unimodal distribution - Mod.CP]. This was because vCJD had been linked to a series of different genes that made people susceptible to the disease. Incubation periods may vary depending on the specific gene combination of each individual.

Source: ProMED.

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