'It's not surprising that some members of the public are left hanging because they're not doing what they should do to look at the issue,' said Sallie Bernard, co-founder of SafeMinds. 'There's research they (the government) could do that specifically looks at autistic kids and mercury burden.'

A day before parents who blame the chemical were to complain to Congress, federal health officials stressed that the only childhood vaccines that still contain the preservative are some, but not all, flu shots -- and that there's no credible evidence that it caused the brain disorder anyway.

But they brought no new data to the unusual gathering that might close the case on thimerosal.

"We don't know, unfortunately, for most kids with autism what causes it," said Dr. Julie Gerberding, director of the Centers for Disease Control and Prevention. "We are committed to getting answers. It's not going to happen overnight."

Hunting the Real Culprit

A scientist who also has a 12-year-old autistic daughter joined the government in urging that the focus shift to hunting the real culprit and a good treatment.

"We need a war on autism, not a war on childhood vaccines," said Dr. Peter Hotez of George Washington University, who said he may rail against his daughter's grueling brain disorder but is sure that it "had absolutely nothing to do with vaccines she received."

Hotez is a microbiologist attempting to develop a vaccine against hookworm, which attacks children in developing countries.

Autism is a complex developmental disorder best known for impairing a child's ability to communicate and interact. Recent data suggest a 10-fold increase in autism rates over the last decade, although it's unclear how much of the surge reflects better diagnosis and how much is a true rise.

Mercury Burden

Thimerosal has been used as a pharmaceutical preservative since the 1930s. Although the amount of mercury it contains is very small, it was phased out of routine child vaccines as of 2001 because of concern about increasing doses as babies received ever-more shots.

Studies tracking thousands of children have found no association between autism and thimerosal, but critics say those studies are flawed.

Today, the last childhood shot to contain the preservative is flu vaccine. This fall, manufacturer Aventis Pasteur will provide 8 million thimerosal-free pediatric flu shots, double last year's amount, a spokesman said Tuesday.

Rep. Dave Weldon, R-Fla., has introduced legislation to speed thimerosal's phase-out from flu vaccine, and parents' groups plan a rally Wednesday to urge more study of thimerosal's past role.

"It's not surprising that some members of the public are left hanging because they're not doing what they should do to look at the issue," said Sallie Bernard, co-founder of SafeMinds. "There's research they (the government) could do that specifically looks at autistic kids and mercury burden."

The National Institutes of Health is funding a California researcher to do that, part of $102 million in autism research this year, said pediatric disease chief Dr. Duane Alexander.