U.S. health officials defended the safety of childhood vaccines Tuesday in an apparent attempt to answer vocal critics questioning the connection between some vaccines and autism.

Officials told reporters that they are worried that concerns over possible connections between autism and the mercury-containing preservative thimerosal are discouraging parents from vaccinating children against a host of infectious diseases.

Their arguments, timed a day before autism and antivaccine activists are scheduled to attend a rally on Capitol Hill, break down along a familiar line: there is no evidence vaccines containing mercury are a direct cause of autism, though the science is still somewhat spotty. And even if some connection is found in the future, the benefits of vaccines in preventing more than a dozen potentially deadly diseases far outweigh the risks.

"The science tells us very clearly that vaccines save lives and protect our children," CDC Director Julie M. Gerberding, MD, told reporters.

Gerberding and other officials stress that childhood vaccines have virtually erased diseases like polio, diphtheria, and measles, that once killed thousands of U.S. children. But such diseases remain common in developing countries where vaccination rates are low, meaning that they continue to pose a potential threat in the U.S.

Read Web MD's "New Intensity to Debate Over Autism Cause."

Rise in Autism Rates

At the same time, scientists still don't know what has caused the rise in diagnosed autism cases over the past 20 years. Between 2 and 6 per 1,000 children born in the U.S. are diagnosed with autism, putting the overall prevalence as high as one in every 163 children.

Some blame the rise on thimerosal, a mercury-containing chemical that was used as a vaccine preservative in childhood vaccines until 2001. The chemical is still used in influenza vaccines and exists in trace amounts in others because manufacturers continue to use it to disinfect equipment.

Public concern over thimerosal has spurred vaccine makers to try to minimize its use. The preservative is needed in multidose vaccine vials, such as the flu vaccine, forcing manufacturers to switch to single-dose vials to avoid using it.

The CDC has signed a contract to purchase 3.5 million doses of preservative-free flu vaccine from manufacturer Sanofi-Pasteur, Gerberding told a House committee in May. The company is expected to make another 3 million to 5 million thimerosal-free doses.

"We are very, very concerned that people have confidence in the products that they use," said Murray Lumpkin, MD, acting deputy commissioner of the FDA.

A 2004 report from the Institute of Medicine rejected any link between thimerosal-containing vaccines and autism.

Read Web MD's "New Vaccines for Old Diseases."

Thimerosal Legislation

Still, many researchers are troubled by evidence that a rise in autism rates appeared to roughly coincide with the use of the chemical in the vaccine supply.

Gerberding said that she "cannot sit here and tell you with 100% certainty" that a link between autism and vaccines will never be found in individual children. "The predominance of evidence does certainly not reveal an association between thimerosal and autism," she said.

Bobbi Manning, vice president of a group known as Advocates for Children's Health Affected by Mercury Poisoning, tells WebMD that officials "are not being completely honest about the effects of mercury."

"It's not enough to hear that they've taken thimerosal out of vaccines. We still have a lot of damaged children," she said.

Rep. David Weldon (R-Fla.), who is an internal medicine doctor, said that he plans to introduce legislation tomorrow gradually banning thimerosal from all childhood vaccines beginning in 2007. Weldon also says he'll push for more government research on the causes of America's rising autism rate.

"We all know that these vaccines save thousands of lives," Weldon told reporters. "I think the incidence [of autism] is definitely up and what's yet to be determined is why that is."

As for the defense of vaccines by health officials on Tuesday, Weldon said: "They just want to reassure people because the press tomorrow is going to be raising a lot of questions about mercury."

Read Web MD's "Use WebMD's Childhood Immunization Calendar."

By Todd Zwillich, reviewed by Michael W. Smith, MD

SOURCES: Julie M. Gerberding, MD, director, CDC. Murray Luskin, MD, acting deputy director, FDA. Bobbi Manning, vice president, Advocates for Children's Health Affected by Mercury Poisoning. Rep. David Weldon (R-Fla.).