A May 19 article about an Institute of Medicine report on vaccines and autism incorrectly characterized Rep. David Joseph Weldon (R-Fla.) as a "vaccine skeptic." Weldon, who is a physician, supports childhood vaccinations and believes their benefits outweigh the risks. But he believes that the federal government has not done enough to study adverse reactions to vaccines, and specifically the hypothesis that the measles-mumps-rubella (MMR) vaccine, or vaccines containing the preservative thimerosal, cause autism. (Published 5/20/04)

The Institute of Medicine, a highly influential adviser of the government on scientific matters, said yesterday there is no credible evidence that either the measles-mumps-rubella (MMR) vaccine or vaccines containing the preservative thimerosal cause autism.

The conclusion came in an 81-page report requested by two federal agencies to address the doubts raised by a small but vocal group of parents who question the safety of childhood vaccines.

A 14-person panel of experts urged more research on autism but said further pursuit of possible links between vaccines and the devastating neurological disorder is probably not worth the money and effort.

Reports published in 2001 by the same committee found no connection between the MMR vaccine and autism, and insufficient evidence to draw conclusions about thimerosal, a mercury-based preservative added to multiple-dose vials of vaccine. Since then, enough new studies have been published to confidently reject both theories, the panel said.

Especially convincing were a Danish study showing no difference in the rate of autism between children who got thimerosal-containing vaccines and those who did not and a British study showing no relationship between the introduction of MMR and autism rates, or between the timing of a vaccination and the onset of autism symptoms.

"The vaccine hypotheses are not currently supported by the evidence," wrote the panel, which consisted of physicians, neuroscientists, epidemiologists, statisticians and a nurse.

In a telephone news briefing, the chairman of the committee, Marie C. McCormick of the Harvard School of Public Health, said her advice to parents is that children "should be getting their vaccines" because the life-threatening infections they protect against "are only a plane ride away."

After decades in which vaccines were generally considered certified miracles of medicine, the preparations and their safety have been openly challenged in recent years as the number of routine childhood immunizations has risen, along with the prevalence of several behavioral disorders.

The debate has been acrimonious. Some vaccine opponents have accused the government of covering up damaging information, while some public health officials are exasperated by what they view as the equal weight being given to small, poorly designed studies supporting the vaccine-autism connection and large, well-designed ones refuting it.

In Britain, public doubts about the MMR vaccine pushed the measles immunization rate from 92 percent in 1996 to 81 percent in 2003, and led to a tripling of measles cases. In the United States, MMR immunization has held steady at about 92 percent from 1998 through 2002.

The Institute of Medicine panel said five observational studies "consistently provided evidence of no association" between thimerosal and autism in Sweden, Denmark, the United States and Britain. A similar number of studies that do suggest a link are small, uninterpretable, unpublished or poorly designed, it said.

Fourteen studies, including nine controlled ones, found no association between the MMR vaccine and autism. Three that found a connection are poorly designed or offer very indirect evidence that could be explained in other ways, the authors said.

Much of the support for the idea that vaccines may cause autism arises from the fact that the neurological illness usually becomes evident after age 3 -- sometimes with dramatic social regression and loss of language -- at a time when children are getting multiple immunizations.

In the case of thimerosal, the underlying biological mechanism was thought to be that some children are unusually sensitive to mercury, a known neurotoxin, even though the amount in shots is minuscule. (Since 2000, thimerosal has been taken out of all routinely given childhood vaccines.)

With MMR, the theory is that the vaccine -- which contains a live, very weakened strain of the measles virus -- promotes the production of neurotoxins, or causes damage to the immune system, which in turn leads to brain damage. An English researcher, Andrew J. Wakefield, found fragments of the measles virus in the intestines of a small number of autistic children with severe digestive problems. Other scientists are only now trying to confirm this observation.

In the telephone briefing, McCormick and Steven Goodman, a Johns Hopkins physician and biostatistician who also served on the panel, did not rule out the possibility that some autistic children may have disorders of immunity or of digestion that explain the findings.

They noted that autism is a neurological disorder, and that the human gut is now known to have its own, largely independent nervous system. Both the brain and the gut might be damaged by the developmental catastrophe that appears to occur before birth, or just after it, and that leads to autism.

One of the nation's leading vaccine skeptics, Rep. David Joseph Weldon (R-Fla.), who is a physician, criticized the report as "premature, perhaps perilously reliant on epidemiology, based on preliminary incomplete information."

Rick Rollens, a former secretary of the California Senate who became an advocate for autism research when his son, now 13, developed the disease, said he thinks "this report represents the will of the powerful public health community, CDC and vaccine manufacturers." He predicted it will not put the issue to rest.

The Institute of Medicine committee has produced eight studies on vaccine safety at the request of the National Institutes of Health and the Centers for Disease Control and Prevention. This is the last one, and the committee will now disband.