Federal health officials affirmed the safety of vaccines on Tuesday in an unusual news conference called to counter a growing movement alleging that vaccines can cause autism.

Autism activists planned a rally in front of the U.S. Capitol on Wednesday to press their contention that the government has covered up evidence linking autism to a mercury-based product once used in vaccines.

They will also demand more research into autism, which they say is a growing problem.

“There has been a renewed interest in the issue of vaccine safety, particularly on the issue of whether the preservative thimerosal ... may be linked to the occurrence of autism in children,” Dr. Julie Gerberding, director of the U.S. Centers for Disease Control and Prevention, told the news conference.

Autism is a mysterious illness that is often diagnosed between the ages of about 18 months and four years - just the time that toddlers and young children are getting their vaccines.

Several reports, including a review of all the studies from the independent Institute of Medicine, have found no evidence that vaccines cause autism.

But some groups of parents allege a cover-up, and some have resisted vaccinating their children - leading to outbreaks of disease such as measles and whooping cough in Britain, Netherlands and elsewhere.

They believe there is evidence that at least some children are sensitive to mercury, including the mercury in thimerosal.

Thimerosal is no longer used in childhood vaccines in the United States.

The CDC officials, as well as representatives of the National Institutes of Health, American Medical Association, American Academy of Pediatrics, and the Food and Drug Administration, made it clear they believed some of the groups were relying on incomplete information.

GETTING A BETTER HANDLE

Gerberding mentioned one report that as many as one in 166 U.S. children have autism, and said more studies were needed to get a “better and more reliable handle” on how many cases there are. Several recent studies have questioned the contention that autism rates have gone up in recent decades.

Said Dr. Eileen Ouellette, president-elect of American Academy of Pediatrics: “We want to discover the causes of autism as well as how to prevent it and treat it, but the evidence does not point to vaccines as one of those causes.”

Even as health officials hoped the debate had finally been laid to rest, it gained new life with a book by former journalist David Kirby alleging a potential link, and a television interview last week in which Robert Kennedy Jr., nephew of slain president John F. Kennedy, alleged a conspiracy.

And the announcement of the news conference raised suspicions among the activists.

“Groups who have set up the march believe this CDC press conference is a shameful attempt to head off publicity from the rally that would direct attention to the role of thimerosal and vaccines in autism,” the South Carolina-based National Autism Association said in an e-mail to supporters and journalists.

Gerberding acknowledged the mistrust.

“Parents want answers,” she said. “But when looking at answers to problems we have to be careful not to base our decisions ... on unproven hypotheses and fear.”

Kirby, who plans to speak at the rally on Wednesday, said he was surprised the CDC held a news conference with no new study to announce.

“I think people at the CDC have not been totally forthcoming about the information they are sitting on. I think the whole story is not getting out there,” Kirby said in a telephone interview.