The government´s push this year to vaccinate young children against the flu has triggered new interest in banning a mercury-based preservative found in most influenza shots.

Called thimerosal, the preservative was used in many children´s vaccines until manufacturers began phasing it out in 1999 amid concerns that mercury might contribute to autism and other developmental disorders.

America´s reduced supply of about 54 million influenza vaccines this flu season still includes 4.6 million shots that are free of thimerosal. The nation´s total supply of almost 100 million doses was slashed nearly in half last week after the British government withheld a major manufacturer´s shipment because of quality concerns.

In the ensuing pinch to get a vaccine in coming weeks, parents might not have the luxury of requesting doses without the preservative. Some are worried about exposing their children to thimerosal in that instance. But most doctors think there´s a much greater risk in foregoing the vaccination.

Influenza caused a startling string of children´s deaths early last season, prompting a run on flu shots and panic among parents in some hard-hit states such as Colorado.

"Get the vaccine," said Dr. Douglas Short, a pediatrician at Arnold Palmer Hospital for Children & Women in Orlando, Fla. "No one has ever been proven to have been harmed by thimerosal, while lots and lots of babies get really sick from the flu every year and a few will die from it. It´s an obvious risk versus a theoretical one."

But even so, some parents, doctors and now, members of Congress, are calling for more research on thimerosal. U.S. Rep. Dave Weldon, R-Fla., thinks the federal government should ban the substance in all vaccines, or at least keep it out of all children´s doses.

He sits on a congressional subcommittee that held a hearing on thimerosal last week in Washington.

Weldon is not convinced by U.S. and European studies that have found no link between thimerosal and autism, nor by the expert panels that have drawn similar conclusions.

He says the studies have been so flawed in their designs or otherwise limited that they are evidence of the half-hearted, dismissive attitude by the scientific community toward the topic.

"My biggest complaint is they have never taken this issue seriously and funded the necessary research" to truly study thimerosal, said Weldon, a medical doctor and father.

Thimerosal contains a little-understood form of mercury called ethylmercury. It has been used in vaccines for more than 50 years as a way to prevent contamination in vials that contain more than one dose. Children´s vaccines are now sold in single-doses, eliminating the need for the preservative.

But influenza shots - which just this year became a routine recommendation for babies age 6-23 months - historically have been focused on adults. Most say thimerosal is not a threat to adults, who are larger in size and fully mature.

Experts say children, with their small bodies and rapid, ongoing development, are more vulnerable to potential toxins such as mercury.

Governmental and professional organizations such as the American Academy of Pediatrics joined in 1999 to support removing thimerosal from children´s inoculations.

The move came after the Food and Drug Administration discovered that some children were receiving enough thimerosal in vaccines during their first six months to exceed EPA limits on a similar form of mercury called methylmercury, which can be found in mercury-contaminated fish.

The EPA doesn磘 have enough data to establish limits for ethylmercury. But its cousin methylmercury has been linked to problems with vision, learning and coordination. Health officials said the decision to remove thimerosal from childhood vaccines was strictly precautionary.

Several large studies within the past couple of years alone have not found any evidence that thimerosal is linked to autism, a complex developmental disorder that affects a child´s ability to communicate and interact with others.

For example, Danish researchers in 2003 found the rates of autism continued to rise in that country even seven years after thimerosal was removed from vaccines.

In May, a panel from the prestigious Institute of Medicine reviewed a number of studies and concluded that thimerosal is not linked to autism, though it did not rule out a connection to other developmental disorders such as speech delay and attention-deficit disorder.

Cindy Smith´s two children went through the regular regimen of childhood vaccinations, but the Longwood, Fla., mother will be passing on the influenza shot this year.

Her plan is to keep Nicholas, 18 months, and Matthew, 5 years old, as healthy as possible with a wholesome diet and overall healthy lifestyle. Knowing that thimerosal might be in the vaccine is one of her reasons.

"If I knew that was in a vaccine, I definitely wouldn磘 do it," Smith said.

Parents who are concerned about the preservative should ask their doctors, who will know if it´s contained in the vaccines they磖e using.

Weldon, who supports vaccinations for children, also faults the government and industry for failing to ramp up production of thimerosal-free doses so that parents don磘 have to worry about them. He said regulators are dragging their feet on the issue because of the implications of problems with thimerosal.

"If this is ultimately proven, this is going to be a huge, colossal liability for a lot of people," he said.

Weldon said he would not want to deter parents from getting children vaccinated. They should get information on the issues and make an informed decision for themselves, he said.

Many pediatricians, the CDC, the FDA and other groups do not hesitate to say that vaccination is the right decision. By the end of last year´s flu season, 153 influenza deaths in children had been reported to the CDC.

The agency can磘 say if that´s a high number or not, because child flu deaths have never been tracked nationally.

Short, from Arnold Palmer, said doctors have noticed increasingly that young children under the age of 2 are among the most vulnerable to flu´s serious complications.

"The rate of hospitalization for young babies is almost as high as the rate for older folks over 65," he said.

The vaccine is not approved for children under 6 months, which is why the government recommends that all caretakers of infants in that age group get vaccinated. If everyone around the baby has been inoculated, the child has less of a chance of picking up the virus.