Mercury-Autism Debate Left Open

There is no evidence to either prove or reject the theory that mercury in childhood vaccines has lead to autism and other developmental disorders, but the theory is “biologically plausible,” a government panel said Monday.

Thimerosol, a mercury compound, is no longer used in new vaccines for children although some may remain in stock in clinics and pharmacies. Until last year the compound was widely used as a preservative to prevent bacterial contamination in some vaccines.

Parents of autistic children and their advocates have been pointing to the use of the compound as a possible cause of an increase in autism cases.

But the Institute of Medicine report, commissioned by federal agencies, found no convincing evidence to support their fears. But neither did it find any evidence to reject the those fears outright. The report called for more research to settle the issue.

“It should be reassuring to parents that we can find no evidence linking thimerosol to any neurological disorder,” said Dr. Marie McCormick of the Harvard School of Public Health, who headed the committee that drafted the report.

“If a vaccine without thimerosol is available, it should be used. However, if that vaccine is not available, it is far better to be vaccinated with a thimerosol-containing vaccine than not to be vaccinated. The risks of not being vaccinated are certain, while those of vaccines are . . . theoretical.”

Thimerosol was never used in the measles-mumps-rubella vaccine, which has been viewed by parents as the major villain in the surge of autism cases. Its primary uses were in vaccines for hepatitis B, hemophilus influenza B, meningitis and diphtheria-pertussis-tetanus. All are now available in thimerosol-free formulations.

Parent advocates reacted favorably to the new report. “They are taking a very strong position compared to other Institute of Medicine reports,” said Portia Iversen, a co-founder of Cure Autism Now. “I just wish they had gone one step further and asked for a recall of all mercury-containing vaccines.”

That sentiment was echoed by Sallie Bernard of the nonprofit Safe Minds. “We believe that no child should get any mercury-containing vaccines,” she said.

But Dr. Louis Cooper of Columbia University, president-elect of the American Academy of Pediatrics, said parents should not be concerned. “No one knows the magnitude of the stocks [of thimerosol-containing vaccines], but my sense of it is that the numbers are likely to be quite small.”

In 1999, federal and private agencies recommended that the preservative no longer be used in children’s vaccines, and that recommendation was widely adopted by the middle of last year.

The principal ingredient of thimerosol is ethylmercury, a chemical cousin of the methylmercury found in mercury-contaminated fish. Ethylmercury was used in multidose vials of vaccines to prevent contamination by bacteria when a syringe needle is injected into the vial repeatedly. Most children’s vaccines now come in single-dose vials and no preservative is required.

The mercury compound is still used in some vaccines for adults, including the influenza vaccine, as well as in some over-the-counter products, such as nose drops. But most physicians have never considered that a problem.

“The issue is really the amount of ethylmercury per body weight, and the dose in an adult is much smaller” than in a child, McCormick said.

The report is available on the Internet at https://www.nap.edu.