While health officials tout studies showing no link between thimerosal and autism, critics allege that thimerosal, a mercury-based preservative that is used to prevent bacteria growth in some multi-dose vials of vaccine, dramatically raises the risk of autism, a little understood mental health disorder that limits a person's ability to interact with the world around them.

But a spokesman for the Public Health Agency of Canada says there is nothing to worry about here. "Most vaccines in Canada do not use this ingredient," explained Julian Beltrame, media relations chief for the Public Health Agency of Canada.

According to the agency, all routine childhood vaccines with the exception of some hepatitis B vaccines are thimerosal-free. Some flu shots also contain this ingredient, but manufacturers are working on a thimerosal-free formulation.

"We are trying to phase it out, not because of scientific concerns but because of public perception," Beltrame said. "Our position is that no credible studies show that there is any link to autism or any other disease."

But those in the anti-vaccine camp say their studies have found the exposure to thimerosal through vaccinations is a "significant risk factor" for the development of neurological disorders.

In any case, because thimerosal is not in most of the vaccines used here, a document from the National Advisory Committee on Immunization says, "the amount of mercury exposure through vaccines for Canadian children, even in jurisdictions where there is a routine hepatitis B infant immunization program, is well below even the most conservatively acceptable and tolerable limits."

As such, public health officials argue that the cost of not vaccinating children outweighs the risks. "Those who do not immunize not only risk the health of their children, but also those with illnesses or allergy who cannot be immunized, and those who were immunized but did not develop immunity," wrote Canada's chief public health officer Dr. David Butler-Jones in an op-ed piece in the Toronto Star.

Entitled "Vaccine myths and why they are dangerous," the article praised the effects of routine vaccination on Canadians' health and cited examples where lapses in vaccinating have lead to outbreaks of disease.

"Perhaps because vaccines have been so successful, many feel they are no longer needed. But these diseases can return if we let down our guard. From 1993 to 1997, there were 5,000 deaths from diphtheria in the former Soviet Union after organized immunization was suspended. In 2003, polio eradication campaigns, which have since resumed, were halted in Nigeria because of false information about oral polio vaccine. Polio has since become re-established in a growing number of countries in sub-Saharan Africa, and has recently spread from Sudan to Yemen and Indonesia. Campaigns against whooping cough vaccine in the United Kingdom, based on false information on the risk of the vaccine, caused immunization rates to fall. This resulted in a return to large scale whooping cough epidemics with far more deaths and brain damage among children than we would want to think of," Butler-Jones wrote.

"These are the facts and it is a needless tragedy when a child dies or is crippled for a lack of a vaccine."