Further Evidence Shows No Link Between Thimerosal and Increased Autism Risk

From Medscape Medical News
Emma Hitt, PhD

September 16, 2010 — Prenatal and infant exposure to thimerosal, a mercury-containing preservative used in some vaccines, does not increase risk of autism spectrum disorder (ASD), new research findings from the Centers for Disease Control and Prevention (CDC) and others suggest.

Frank DeStefano, MD, MPH, the CDC's director of immunization safety, and colleagues reported the findings in the September 16 issue of the New England Journal of Medicine.

According to the researchers, in 1999, the US Food and Drug Administration estimated that infants who were immunized according to the recommended schedule might have received amounts of ethylmercury in excess of Environmental Protection Agency limits for exposure to methylmercury. As a result, vaccine manufacturers were urged to remove thimerosal from all infant vaccines and to investigate any potential risks.

In the current study, Dr. DeStefano, in collaboration with researchers from Kaiser Permanente; Harvard Medical School, in Boston, Massachusetts; Stanford University, in Palo Alto, California; and others reviewed records from a managed care organization. They also interviewed the parents of 256 children verified to have ASD according to a standardized personal evaluation.

Children with ASD were further classified into having autistic disorder or ASD with regression. The children with autism were matched by age, sex, and managed care organization to children without autism.

None of the autism outcomes were associated with prenatal or early-life receipt of thimerosal-containing vaccines and immunoglobulins, and the receipt of vaccines was comparable between patients with ASD and control patients.

The adjusted odds ratios for ASD associated with a 2–standard deviation increase in ethylmercury exposure were 1.12 (95% confidence interval [CI], 0.83 - 1.51) for prenatal exposure, 0.88 (95% CI, 0.62 - 1.26) for exposure from birth to 1 month, 0.60 (95% CI, 0.36 - 0.99) for exposure from birth to 7 months, and 0.60 (95% CI, 0.32 - 0.97) for exposure from birth to 20 months, thus representing a decrease in risk for some time periods.

The researchers note that they are "not aware of a biological mechanism that would lead to this result."

Parents, Physicians Should Be Reassured

Dr. DeStefano told Medscape Medical News that this study adds stronger evidence regarding exposure during pregnancy and is the first study to evaluate subtypes of ASD, including ASD with regression.

"These findings add to the evidence that thimerosal-containing injections during pregnancy or infancy do not increase a child's risk of developing autism," he added. "In my opinion, further studies are not warranted," he said.

According to Dr. DeStefano, the Institute of Medicine has also reviewed the evidence regarding thimerosal and autism and concluded that it did not support such an association, and "the results of our study add further support to this conclusion."

"Clinicians can feel confident in counseling parents that the scientific evidence does not support a link between thimerosal in vaccines and autism," Dr. DeStefano said. "Currently, this should not be much of an issue because thimerosal is no longer included in most routinely used in infant and childhood vaccines in the United States."

According to Dr. DeStefano, the single exception is in some influenza vaccines, but "thimerosal-free preparations of influenza vaccines are available for parents who are more comfortable with thimerosal-free products."

Independent commentator Meg Fisher, MD, chair of the Department of Pediatrics and Medical Director with the Children's Hospital at Monmouth Medical Center, in Long Branch, New Jersey, noted that this study confirms previous findings that show no association of autism and thimerosal.

"It was a well-done study using information from 3 managed care organizations," she told Medscape Medical News. "Exposure to thimerosal through vaccines and immunoglobulin was determined both after birth and prenatally, and there was no increased risk of ASD associated with exposure to ethylmercury — prenatally or postnatally."

Dr. Fisher concurs with Dr. DeStefano that additional studies on this issue are not needed. "Parents and physicians should be reassured by this additional, well-designed study — there is no association of thimerosal and autism."

She added that the CDC's Vaccine Safety Datalink system provides a wealth of clinical data that can be used to test hypotheses about vaccine adverse events.

Dr. Marcy received honoraria for speaking for Merck and GlaxoSmithKline within the last 5 years and grant support for studies on Gardasil and ProQuad from Merck within the last 5 years; Mr. Lewis received grant support from Medimmune, Sanofi Pasteur, Chiron, Wyeth, Merck, and GlaxoSmithKline; and Dr Bernal received research funding from the CDC, the National Institute of Mental Health, Health Resources and Service Administration, and Autism Speaks. The other authors have disclosed no relevant financial relationships.

Pediatrics. 2010;126:656-664.

Disclosure: Emma Hitt, PhD, has disclosed no relevant financial relationships.