As time has gone on, it has become clear that this worst-case scenario is not going to happen. To date, 154 people have died of vCJD, and six are ill. The number of deaths reported annually has fallen steadily since a peak of 28 in 2000, with five last year and one so far in 2006. Last year the most definitive epidemiological research, by Paul Clarke and Azra Ghazi of Imperial College, London, predicted that no more than 70 more people were likely to die. Another 3,000 are probably infected, but will die of other causes before showing symptoms of the brain-wasting disease.

The Lancet study fits squarely within this picture, but adds a new twist. The mouse experiments appear to confirm that, for most people, the incubation time for vCJD is so long that they are unlikely to develop the condition even if they are infected with the rogue prion proteins that cause it. The research, however, also shows that people with all three of the genetic variations that affect susceptibility to vCJD can be infected, even though only one seems vulnerable to the clinical disease. The implication is that the disorder is almost certain to cause a few new infections every year.

This highlights the urgency of developing techniques for screening blood products and donors. There is also a strong case for conducting post-mortem examinations on people who die of causes other than vCJD, to determine whether they were infected with rogue prions without symptoms.

ARE YOU AT RISK?

• People are at risk of developing vCJD if they ate British beef or beef products before 1989, when brain and spinal cord tissue were banned from human food
• Some experts say the risk continued until 1995, when mechanically recovered beef was taken out of the food chain
• A small risk also exists for anyone who has had a blood transfusion in Britain
• There is an even smaller risk to those who have been operated on with non-disposable surgical instruments, as these cannot be completely sterilised of prions

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