HOUSTON—The imminent release of a vaccine to combat the H1N1 ( swine) influenza has stirred up discussion of a decades old debate: will that vaccine be safe if it contains a common flu vaccine ingredient called thimerosal.

Experts say that thimerosal is a preservative. It has been used since the 1930s to prevent contamination in multi-dose vials of vaccines. Thimerosal contains 49 percent ethylmercury, and has been phased out of use in most vaccines except in the case of influenza.

But some researchers, and thousands of parents across the U.S., believe there is a link between vaccines, thimerosal and the onset of autism.  

The Centers for Disease Control has fought that argument for years.  In 1999, the Public Health Service (PHS) agencies, the American Academy of Pediatrics (AAP) and vaccine manufacturers agreed that thimerosal should be reduced or eliminated in vaccines as a precautionary measure.

In 2006, the CDC published a list of talking points on the issue saying, “The amount of mercury found in influenza vaccine has not been found to be associated with autism or any other harm.”

Still, there are parents like Sarah Harm who remain wary and unconvinced.

“No chemicals. No preservatives. Nothing of that nature,” said Harm.

She said that she pledged to never give her daughter vaccines believing the potential harm outweighs the benefits.

“If it was truly thimerosal free, not just marketed as such, perhaps. If it is absolutely necessary,” she said.

But Dr. Paul Glezen, with Baylor’s Department of Molecular Virology and Microbiology and involved in Baylor’s H1N1 vaccine clinical trial does not believe thimerosal has ever been a threat to anyone.

“No it’s not a concern anyway because there’s been no evidence that it produces any harm. It is not stored in the body like metal mercury is which has been shown to be toxic,” he said. “So this is an ethylmercury preparation and is rapidly metabolized and excreted in the body.”

However, Glezen said that if parents do have concerns about the preservative that they request thimerosal-free influenza vaccines. He said they are readily available, and versions of an H1N1 vaccine are expected to be available thimerosal-free as well.

Baylor College of Medicine is in its second month of clinical trials for the H1N1 (swine) influenza vaccine.

The campus is one of eight federally funded centers involved in a series of studies to determine the effectiveness of several experimental vaccines to protect against the H1N1 virus. A workable vaccine is expected to be available next month.