Infant and Environmental Exposures to Thimerosal and Neuropsychological Outcomes at Ages 7 to 10 Years
CDC conducted the Infant and Environmental Exposures to Thimerosal and Neuropsychological Outcomes at Ages 7 to 10 Years study to investigate possible associations between prenatal and early childhood exposure to thimerosal-containing vaccines and/or immunoglobulins and deficits in neuropsychological functioning. Thimerosal is a preservative that contains ethyl mercury and is used in some vaccines and immunoglobulins. This study was not designed to assess possible association between thimerosal and autism.
This study was designed to improve upon previous studies that assessed the possible association between ethyl mercury exposure from thimerosal-containing vaccines and neuropsychological functioning including an objective measurement of neuropsychological functioning. These improvements reduced the potential for study biases.
The study was further strengthened by engaging a panel of independent external consultants to advise on the study design, selection of the neuropsychological test battery, interpretation of results, and editing of the draft manuscript. The consultants have expertise in the fields of toxicology, epidemiology, biostatistics, vaccine safety, and also included a representative from the autism advocacy community. The external consultants provided their individual input into the study protocol and the analysis plan.
The study evaluated 1,047 children between the ages of 7 to 10 years who received vaccines during the 1990s when thimerosal was used as a preservative in many childhood vaccines. The study used a retrospective cohort design. Computerized medical records were used to select a sample of children with a wide range of exposures from thimerosal-containing vaccines and immunoglobulins during infancy. Each child's level of exposure to ethyl mercury was determined through a review of electronic immunization registries, medical records, personal immunization records, and parent interviews. Each child was then administered a series of standardized neuropsychological tests in a clinical setting at ages 7 to 10 years.
The study found only a few statistically significant associations between exposure from thimerosal and neuropsychological functioning. The weight of the evidence from this study does not support an association between early ethyl mercury exposure from thimerosal-containing vaccines and/or immunoglobulins and neuropsychological functioning at ages 7 to 10 years.
The following technical reports are extended versions of the manuscript and describe all of the various analyses conducted in this study. These documents allow for a more complete presentation of methods and results than was possible within the space constraints of the scientific article, "Early thimerosal exposure and neuropsychological outcomes at 7 to 10 years,"* by Thompson et. al.
The Public Use Data Set from the Study of Prenatal and Infant Exposure to Thimerosal and Neuropsychological Outcomes at Ages 7–10 Years is available by mail request through CDC's Immunization Safety Office.
Thimerosal (ethyl mercury) is a preservative that has been added to some vaccines because it is effective in killing bacteria and in preventing bacterial contamination.
Methyl mercury is a chemical that is often found in some fish and exposure primarily occurs by eating fish.
Neuropsychological testing is a procedure that measures and identifies cognitive impairment and functioning in children and adults.
Early childhood is the period between birth and seven months (1 to 214 days).
Frequently Asked Questions
What is the purpose and design of this study?
This study is a follow-up study to the Vaccine Safety Datalink (VSD) thimerosal screening analysis (Verstraeten et. al., 2003), and addresses inconsistent results from that screening study regarding associations between neuropsychological delays and thimerosal exposure. This study examines associations between thimerosal exposure through vaccination and immunoglobulin preparations and the following neuropsychological outcomes:
This study recruited children from May 2003 to May 2004 who, in their first year of life, could have been immunized with vaccines containing thimerosal. The children were between the ages of 7 and 10 years at the time of the study. The study design included the administration of a battery of standardized neuropsychological tests administered by trained testers who were blinded to the children’s thimerosal exposure; the addition of medical chart abstractions and immunization records provided with the permission from biological mother to ensure complete and accurate immunization data; and the collaboration of external expert consultants in development of the study protocol, monitoring of study conduct, interpretation of findings, and review of final publication.
Will this study answer questions about a possible association between thimerosal exposure through vaccination and autism?
This study does not assess autism as a possible outcome of thimerosal exposure through vaccination. Autism occurs too infrequently to be assessed with the study design used in this neurodevelopmental study. CDC is conducting a separate study to specifically evaluate possible associations between thimerosal and autism.
What methods were used for this study?
This retrospective cohort study was performed with extensive assessment of thimerosal exposures and neuropsychological functioning. This included a rigorous review of computerized and paper medical records, personal immunization records, and maternal interviews to assess exposures to thimerosal through vaccines and immune globulins.
To assess neuropsychological functioning, a battery of tests was administered by trained assessors in a clinical setting. The outcomes measured included speech and standard language, verbal memory, achievement, fine motor coordination, visual spatial ability, attention and executive functioning, behavior regulation, tics, and general intellectual functioning. In addition, information was collected from parents and teachers about attention, hyperactivity and executive functioning.
The safety concerns regarding thimerosal exposure is based upon the hypothesis that cumulative exposures to ethyl mercury through vaccines could result in neuropsychological disorders and deficits. Biological samples for the participants were not taken for this study. Rather, the source of the child’s ethyl mercury exposures were obtained through parent interviews, review of medical records and immunization registry information. Because infants excrete significant amounts of mercury in their stool after vaccinations, biological samples obtained at ages 7 to 10 years would not have provided more accurate information.
How was this study design strengthened over earlier studies?
This study employed several methods to address limitations identified in earlier thimerosal studies:
What were the limitations to this study?
The major limitations include:
What are the major findings from this study?
A total of 378 statistical tests were conducted. Each child was tested on 42 neuropsychological outcomes, 3 exposure periods and the full model plus gender specific analyses. Among the 42 outcomes measured, the study found that the majority of the outcomes had NO association with thimerosal exposure and most associations would be what is expected by chance alone.
Only a few statistically significant associations or consistent patterns between exposure to thimerosal and neuropsychological functioning were found.
The weight of the evidence in this study does not support a causal association between early mercury exposure from thimerosal-containing vaccines and/or immunoglobulins and neuropsychological functioning at ages 7 to 10 years.
Who are the expert consultants CDC is collaborating with on this study?
The independent consultants represent a broad spectrum of expertise in fields related to child development, immunization, and immunization safety, as well as a representative from the autism advocacy community. The resulting study is better informed by this broad expertise, and potentially more credible to the larger public because of the external input.
The consultants provided input into the development and revision of the study protocol and analysis plan, and many of the consultants volunteered to provide comments on the study results and the final manuscript.
Public Use Data Set
The public use data set is available by mail request through CDC's Immunization Safety Office. It consists of preliminary and final data files, documents, and reports.
The data collected for this study are protected under an Assurance of Confidentiality and the Data Sharing and Use Agreement (PDF-20KB).
Mail the completed, signed, and dated Data Sharing and Use Agreement to—
Public Use Data Set
The following documents describe the Public Use Data Set:
Questions & Answers: Thimerosal in Seasonal Influenza Vaccine
NIAID-Supported Studies on Mercury, Thimerosal, and Vaccine Safety
Institute for Vaccine Safety*
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Page last modified: September 24, 2008
Content source: Immunization Safety Office