A new study entitled “Early Thimerosal Exposure and Neuropsychological Outcomes at 7 to 10 Years” has been published in the New England Journal of Medicine, vol. 357 no. 13.
It has been hypothesized that early exposure to thimerosal, a mercury-containing preservative used in vaccines and immune globulin preparations, is associated with neuropsychological deficits in children.
We enrolled 1047 children between the ages of 7 and 10 years and administered standardized tests assessing 42 neuropsychological outcomes. (We did not assess autism-spectrum disorders.) Exposure to mercury from thimerosal was determined from computerized immunization records, medical records, personal immunization records, and parent interviews. Information on potential confounding factors was obtained from the interviews and medical charts. We assessed the association between current neuropsychological performance and exposure to mercury during the prenatal period, the neonatal period (birth to 28 days), and the first 7 months of life.
Among the 42 neuropsychological outcomes, we detected only a few significant associations with exposure to mercury from thimerosal. The detected associations were small and almost equally divided between positive and negative effects. Higher prenatal mercury exposure was associated with better performance on one measure of language and poorer performance on one measure of attention and executive functioning. Increasing levels of mercury exposure from birth to 7 months were associated with better performance on one measure of fine motor coordination and on one measure of attention and executive functioning. Increasing mercury exposure from birth to 28 days was associated with poorer performance on one measure of speech articulation and better performance on one measure of fine motor coordination.
Our study does not support a causal association between early exposure to mercury from thimerosal-containing vaccines and immune globulins and deficits in neuropsychological functioning at the age of 7 to 10 years.
Read Early Thimerosal Exposure and Neuropsychological Outcomes at 7 to 10 Years (PDF copyright New England Journal of Medicine).
Read a statement from Autism Speaks regarding the study.
Read a press release from SafeMinds in response to the study.
See news coverage from NBC Nightly News, The New York Times, The Los Angeles Times, Newsweek, USA Today and the Associated Press
Archived | Study fails to link vaccine chemical, brain woes
A large study has found no link between babies’ exposure to a controversial mercury-based vaccine preservative and the development of problems in language, behavior or intelligence.
Advocacy groups who believe the preservative thimerosal is linked to autism or other brain deficits remain unconvinced, calling the study biased and poorly done.
Researchers with the Centers for Disease Control and Prevention and colleagues reporting in today’s New England Journal of Medicine did not look at autism, because another CDC study on that topic is underway, and results are expected next year.
Instead, they gave performance tests to 1,047 children ages 7 to 10 for whom they had lifetime vaccination records and tracked how increasing exposure to thimerosal correlated with neuropsychological outcomes, such as IQ, fine motor skills and language.
Five percent, or 19 of 378 statistical test scores analyzed, showed results considered significant, but 5% is the number that can be predicted to result from chance alone in this size sample, says Anne Schuchat, director of the National Center for Immunization and Respiratory Diseases at the CDC. The significant links went in both directions. Twelve indicated thimerosal exposure improved test performance, while seven showed it resulted in poorer performance.
Only one finding raised a question that researchers say needs further evaluation: a twofold greater risk of verbal and motor tics seen in boys who had the highest thimerosal exposure between birth and 7 months, compared with boys with the lowest exposure.
A link to tics has been noted in two previous studies, so this finding will be studied further. Schuchat says the tics were seen only by trained evaluators and not reported by parents, suggesting they could be transient tics, which are not uncommon in this age group.
Thimerosal has been used in vaccines since the 1930s, but starting in 1999, it has been phased out of most shots given to children. It is still in some formulations of flu vaccine, though thimerosal-free flu shots are available for children.
Several studies have found no ill effects from its use, but concerns about its potential links to autism persist. In an accompanying editorial, Paul Offitt, chief of the Division of Infectious Diseases at Children’s Hospital of Philadelphia, says the report is the “most comprehensive to date” to look at thimerosal and neurodevelopment, and it shows “no evidence of neurologic problems in children exposed to mercury-containing vaccines or immune globulins.”
But Sallie Bernard of the anti-mercury advocacy group SafeMinds, a member of a panel of external consultants who helped design the study and review results, criticized it as coming to conclusions that don’t fit the findings. “The way the study was written was biased,” Bernard says.
“There are some red flags on tics and language and behavioral control that are concerning and suggest a need for further research.”
Fact: Vaccines Do Not Cause Autism.