Are Thimerosal-Containing Vaccines Safe?
Abstract & Commentary
By Toshiki Takenouchi, MD, and Barry Kosofsky, MD, PhD. Dr. Takenouchi is Resident in Pediatric Neurology, NewYork-Presbyterian Hospital; Dr. Kosofsky is Professor of Pediatrics, Neurology, and Neuroscience, Weill Cornell Medical College; Chief of Pediatric Neurology, NewYork-Presbyterian Hospital Dr. Takenouchi and Dr. Kosofsky report no financial relationships relevant to this field of study.
Synopsis: The most comprehensive study published to date did not find evidence of neuropsychologic problems in children ages 7-10 who were exposed to mercury-containing vaccines or immune globulins administered prenatally or during the first 7 months of life.
Source: Thompson WW, et al. Early thimerosal exposure and neuropsychological outcomes at 7 to 10 years. N Engl J Med 2007;357:1281-1292.
In this first large scale U.S.-based study, 1047 children between the ages of 7 and 10 were identified from four HMOs that participated in the CDC's Vaccine Safety Datalink cohort. They were enrolled and administered 42 carefully selected, standardized tests of neuropsychological outcomes. Exposure to mercury from thimerosal was determined from detailed computerized immunization records, medical records, personal immunization records, and parental interviews. Utilizing these extensive databases, the investigators identified the exact timing and magnitude of exposure to ethyl mercury (thimerosal) for each subject during the prenatal period, neonatal period (birth to 28 days), and first 7 months of life. Controlling for multiple relevant maternal, subject, and home environment variables, these investigators assessed whether there was a causal association between the timing or calculated magnitude of exposure to ethyl-mercury and neuropsychological performance on 42 measures. These measures included assessments of speech and language, verbal memory, achievement, fine motor coordination, visuospatial ability, attention and executive functioning tasks, behavioral regulation, tics, and general intellectual functioning. Among the multitude of neuropsychological outcomes, there were only a few significant associations attributable to exposure to mercury from thimerosal. A summary follows. Less than 11% of children were exposed to thimerosal prenatally through maternal vaccination or from receipt of immune globulins (mainly Rh immune globulins), and 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. From birth to 28 days, the median cumulative exposure to mercury from thimerosal was 12.5 micrograms, and increasing mercury exposure was associated with poorer performance on one measure of speech articulation and better performance on one measure of fine motor coordination. From birth to 7 months the median cumulative exposure to mercury from thimerosal was 112.5 micrograms, and increasing levels of mercury exposure were associated with better performance on one measure of fine motor coordination and on one measure of attention and executive functioning. The detected associations were small and almost equally divided between positive and negative effects, with no consistently demonstrable pattern of deficits emerging.
Commentary
Thimerosal, an ethyl-mercury-containing preservative, has been used in vaccines and immune globulins to prevent bacterial contamination since the 1930s. In 1999, the Food and Drug Administration (FDA) estimated that infants who were immunized according to the recommended schedule could receive amounts of mercury exceeding the limits set by the EPA. However, those calculations were based on exposure limits felt to be safe for methyl-mercury, an environmental toxin found in contaminated fish, not ethyl-mercury, a much more rapidly excreted, and presumed safer additive utilized to stabilize vaccines. Eventually, the CDC and FDA decided to exercise precautionary principles, and asked pharmaceutical companies to remove thimerosal from vaccines as quickly as possible. Although there is no harm in exercising such caution, the ensuing belief that thimerosal causes autism escalated to a national panic, and culminated in the recently concluded "Omnibus Autism Proceeding."1 Those hearings were a component of the much needed vaccine injury compensation program, initiated in 1988, to protect the ever-decreasing number of vaccine-making companies from frivolous litigation.
The results of this study, which confirm the findings of two large studies previously conducted in the United Kingdom,2,3 reassure us that there is no clear association between early thimerosal exposure and neurodevelopmental problems as assessed by these instruments. Specifically, this study performed an extensive and comprehensive battery of 42 neuropsychological assessments on more than 1000 patients ages 7-10, and did not detect a consistent pattern of deficits in exposed offspring. A parallel case-control study of autism in relation to mercury exposure is currently underway at the CDC, which should finally "clear the air" regarding the safety of thimerosal.
Although the randomized controlled trial is generally considered the "gold standard" to support medical decision making, existing vaccination programs have precluded such study design. As a developed society, we are obliged to protect our newborn population from the ravages of common devastating communicable diseases. However, we are additionally obliged to ensure that such preventative measures are delivered according to the highest possible safety standards. Therefore, while utilizing epidemiologic studies to test hypotheses regarding a potential association between thimerosal and neurodevelopmental problems, it is critically important that any resulting correlations be examined carefully. Associations do not imply causality, as epidemiological studies are inherently subject to many potential biases and confounders that may affect the validity of the results reported. In this study, there was a careful selection and definition of the study groups, meticulous definition of the timing and extent of exposure to ethyl-mercury, application of rigorous statistical and analytic methods, and cautious interpretation of the outcomes reported. The authors identify that their study did not specifically target autistic spectrum disorder (ASD) as an outcome, and acknowledge that the neuropsychological assessments used in this study might not be very sensitive for ASD. Nonetheless, as a result of this well designed and implemented study, parents and vaccine policy makers can sleep well at night knowing that prenatal and postnatal exposure to ethyl-mercury does not have an adverse impact on an extensive repertoire of neuropsychologic functions assessed in 7- to 10-year-old children born in the United States through the late 1990s, who were exposed to the full series of recommended DTP, Haemophilus influenzae type b, and hepatitis B vaccine regimens.
References
1. http://www.uscfc.uscourts.gov/docket-omnibus-austism-proceeding. Accessed 4/15/08.
2. Andrews N, et al. Thimerosal exposure in infants and developmental disorders: a retrospective cohort study in the United Kingdom does not support a causal association. Pediatrics 2004;114:584-591.
3. Heron J, et al. Thimerosal exposure in infants and developmental disorders: a prospective cohort study in the United Kingdom does not support a causal association. Pediatrics 2004;114:577-583.
The most comprehensive study published to date did not find evidence of neuropsychologic problems in children ages 7-10 who were exposed to mercury-containing vaccines or immune globulins administered prenatally or during the first 7 months of life.You have reached your article limit for the month. Subscribe now to access this article plus other member-only content.
- Award-winning Medical Content
- Latest Advances & Development in Medicine
- Unbiased Content