WHEN wheelchair-bound Michelle Cedillo was pushed into the US Court of Federal Claims last week, the stage was set for a drama that looks like a simple clash between scientific reason and irrationality. Michelle has severe autism. She cannot speak. She beats herself on the chest and finds any new situation disturbing. To her parents, the cause of these symptoms is not in doubt: they say they are a direct result of a mercury-containing compound called thimerosal (or thiomersal in the UK) added as a preservative to the vaccines given to Michelle as a baby (see 鈥淯S vaccines on trial over link to autism鈥�).
Vaccination is recognised in the US as a public good, and in those rare instances where it causes harm, parents can claim compensation for their child. To win, the Cedillos will have to clear a legal bar that is set unusually low: they will have to prove only that it is more likely than not that Michelle鈥檚 autism is the result of her inoculations. Even so, they will have a tough time making their claim stick.
There are two main strands in the evidence usually advanced for the mercury hypothesis, and neither stands up to scientific scrutiny. One centres on the need to explain the present 鈥渁utism epidemic鈥� in the US and Europe, where the prevalence of autism has leapt, perhaps tenfold over the past 20 years. Yet there is no need to invoke mercury as an explanation. That鈥檚 because the number of conditions labelled as 鈥渁utism鈥� has expanded dramatically over the same period. Children with milder forms of the condition, who might not have been diagnosed in the past, are now classed as having an autistic spectrum disorder. Most epidemiologists think this change accounts for most, if not all, of the rise in cases (New Scientist, 13 August 2005, p 37).
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The Cedillos鈥� lawyers will also struggle to present biological evidence to link mercury and autism. They are likely to cite studies suggesting that autistic children have unusually high levels of mercury in their bodies and others showing that mercury can harm brain cells. These may be suggestive, but are no proof of a link. Children with severe autism often have gastroenterological problems that affect the way their bodies process many substances, of which mercury is only one. That does not mean these substances cause autism. And while mercury is a neurotoxin, the tiny quantities in Michelle鈥檚 vaccines have never been shown to cause brain damage. After mercury was removed from childhood vaccines in 1999, no change in the prevalence of autism was seen.
On the surface then, this looks like a battle between the reasoned arguments of experts and irrational parents. This is how health officials have interpreted vaccine disputes in the past, but in so doing they alienated the people they are meant to be advising.
In the UK, a similar debate kicked off in 1998, when scientist and doctor Andrew Wakefield cast doubt on the safety of the measles, mumps and rubella (MMR) vaccine. The response of the medical establishment was well intentioned but disastrous. Experts met behind closed doors and emerged to tell the public the vaccine was safe. Leaflets gave celebrity endorsements of MMR. Rather than examine Wakefield鈥檚 claims, which were shaky at the time and are now widely discredited, the government merely told parents not to be silly. Not surprisingly, parents did not buy it. Take-up of MMR fell from 92 to 82 per cent, close to the minimum level needed if isolated cases are to be prevented from developing into epidemics.
鈥淭he medical establishment鈥檚 response to doubts over the MMR vaccine was well intentioned but disastrous鈥�
Vaccination ranks alongside improved sanitation as one of the great lifesaving technologies. Rejecting it seems crazy. But worried parents care little for medical history. As research into the MMR controversy shows, parents frame vaccine questions in terms of their child鈥檚 safety. Epidemiological evidence matters to them, but not as much as a detailed discussion of the direct benefits and possible harm to their own child.
Public health experts should not ignore the perception that vaccines and autism are linked, however flawed it may be. Instead they must be ready to address parents鈥� concerns and bring science to bear to explain why tales of 鈥渃hild has vaccination, child develops autism鈥� do not mean 鈥渧accination causes autism鈥�. This involves being open, and admitting what science does not know. It takes time and effort, but the alternative is that parents and health officials talk past each other. That risks undermining confidence in vaccines and causing an even greater tragedy.
