WASHINGTON — There is no evidence that a controversial mercury-based vaccine preservative causes autism, concludes an eagerly anticipated scientific review that says it's time to lay vaccine suspicions to rest and find the real culprit.

Tuesday's conclusion by the Institute of Medicine was a blow to parents of autistic children who blame vaccination for the brain disorder and are pushing for more research of the issue.

Critics said the final proof may come if autism diagnoses drop now that the once-common preservative thimerosal has been virtually eliminated in routine childhood vaccines.

But the Institute of Medicine's panel of prominent scientists pointed to five large studies, in the United States, Denmark, Sweden and Britain, that tracked thousands of children since 2001 and found no association between autism and thimerosal.

While high doses of mercury can cause neurologic damage, there's no evidence that this type of damage causes the symptoms specific to autism - and no laboratory or animal research that proves how the much smaller amounts in thimerosal could do so, either, the IOM concluded.

On the other hand, genetics play a role in autism, and several studies show clear signs of prenatal onset of the disorder, including brain differences at birth, the report notes.

"Don't misunderstand: the committee members are fully aware that this is a very horrible and devastating condition," said Dr. Marie McCormick, a Harvard professor of maternal and child health who led the IOM probe. "It's important to get to the root of what's happening."

But "there seem to be lots of opportunities for research that would be more productive" than continuing the vaccine hunt.

Autism is a complex developmental disorder best-known for impairing a child's ability to communicate and interact with others. Recent data suggest a 10-fold increase in autism rates over the last decade, although it's not clear how much of the apparent surge reflects better diagnosis and how much is a true rise.

Thimerosal has been used as a pharmaceutical preservative since the 1930s. Although the amount of mercury it contains is tiny, public health officials ordered manufacturers in 1999 to phase thimerosal out of common vaccines, from hepatitis to diphtheria, as a precaution, saying small infants had begun getting so many immunizations that they might get too much of the chemical.

Today, with the exception of some - not all - flu vaccines, none of the vaccines used to protect preschoolers in the United States against 12 infectious diseases contain thimerosal, the government says.

Thimerosal critics, who had derived hope from a 2001 Institute of Medicine review that called the potential link unproven but medically plausible, were disappointed by Tuesday's reconsideration.

"The science is still out, the verdict is still out," said Lyn Redwood, president of the SafeMinds activist group and mother of an autistic son. "They've just set us back five years in terms of getting more science."

Some parents argue that children may be born genetically susceptible to autism and some environmental factor, such as thimerosal, triggers it. Large population studies wouldn't catch those subsets of vulnerable children, said Redwood, who had urged the IOM to await some additional research, including studies of genetically susceptible mice, before reconsidering the issue.

But Dr. William Schaffner, a well-known infectious disease specialist at Vanderbilt University, said while the controversy is too emotional to soon disappear, it has received "a terrific amount of research" and the institute's report settles the issue for most scientists.

"It says . . . we need not divert energy and financial and scientific resources to go further down what is a scientific blind alley," Schaffner said. "We need to address the issues of importance to the whole field of autism."