Tempest in a flu shot

Flu vaccine preservative thimerosol's alleged link with autism hits the headlines — again

By Giancarlo La Giorgia

"The study provides no information on how much, if any, thimerosal in vaccines actually reaches the brain in a child. The results are of no help whatsoever in determining whether there is any relationship between vaccination and autism," writes Dr Ronald Gold and Dr Joanne Embree in a letter to the editor of the Ottawa Citizen.

The two eminent pediatricians are riled up about a study published in the January online issue of Molecular Psychiatry, which links the vaccine preservative thimerosal (which is half mercury by weight) and childhood neurodevelopmental disorders like autism and attention-deficit hyperactivity disorder. Dr Gold, a retired University of Toronto pediatrics professor and author of Your Child's Best Shot: A parent's guide to vaccination, and Dr Embree, chair of the Canadian Paediatric Society's Infectious Diseases and Immunization Committee, were moved to write their letter early in February following the furor in the press over the study.

"There's no question that mercury, if present in sufficient concentrations, can damage brain cells, [but] there really is no good clinical or epidemiological data to support any increased risk with vaccination," says Dr Gold.

THE STUDY
Dr Richard Deth, one of the study's authors, couldn't agree less. Although the findings run counter to mainstream medical opinion, Dr Deth, professor of pharmacology at Boston's Northeastern University, strongly believes he and his colleagues have in fact found a link. "I'm aware of the epidemiological studies that have failed to find statistical associations between thimerosal and autism... ," he says. "Our work stands at the other end of the spectrum because it's a molecular study, at the smallest level of inquiry about what it is that the molecule thimerosal and its ethylmercury payload does to nerve cells. It very potently inhibits a [neurological] process that is related to the symptoms of autism, as strongly or stronger than other known causes of developmental disorders, like lead poisoning or alcohol consumption during a pregnancy."

CANADIAN THREAT?
Thimerosal has been added to most multi-dose vaccine vials since the 1930s to prevent contamination on repeated use. As recently as March 2003, Health Canada stated that thimerosal-containing vaccines are perfectly safe, because the preservative's concentration is only around 25g per 0.5mL of vaccine. That said, all routine childhood vaccines except the flu vaccine have been thimerosal-free in Canada since 2001.

Dr Monika Naus, chairperson of Health Canada's National Advisory Committee on Immunization, believes the main motive for switching to more costly, thimerosal-free vaccines was public perception. "We do many things in this country based on the level of concern among the public that may not be cost-effective or highly evidence-based. There's often a response to try and minimize even theoretical risks," she says.

"The concerns that parents have about mercury exposure are not going to go away, regardless of the evidence," says Dr Naus regretfully. "There are always segments of the population who don't get vaccinated at all, for a variety of concerns not based in science." But she urges doctors to reassure parents, including those of autistic children, that the benefits of vaccination outweigh the risks.

Further reading: Health Canada info on thimerosal: http://www.hc-sc.gc.ca/ (enter 'thimerosal' in search field). For a more critical point of view: http://www.test foun dation.org/thimerosal.htm