Exposure to thimerosal, a preservative that contains ethylmercury, during childhood is not a primary cause of autism.
This is the conclusion of a study published in the January Archives of General Psychiatry (Vol. 65, no. 1) by Robert Schechter, MD, MSc, Immunization Branch and California Center for Autism and Developmental Disabilities, Research and Epidemiology, and Judith K. Grether, PhD, Environmental Health Investigations Branch, of the California Department of Public Health, Richmond. Schechter’s and Grether’s article is entitled Continuing Increases in Autism Reported to California’s Developmental Services System: Mercury in Retrograde. By studying time trends in the Archives of General Psychiatry (Vol. 65, no. 1) by Robert Schechter, MD, MSc, Immunization Branch and California Center for Autism and Developmental Disabilities, Research and Epidemiology, and Judith K. Grether, PhD, Environmental Health Investigations Branch, of the California Department of Public Health, Richmond. Schechter and Grether’s article is entitled Continuing Increases in Autism Reported to California’s Developmental Services System: Mercury in Retrograde. By studying time trends in the prevalence by age and birth cohort of autistic children who were enrolled in the California Department of Developmental Services (DDS) from January 1, 1995 through March 31, 2007, the authors found that
“the estimated prevalence of autism for children at each year of age from 3 to 12 years increased throughout the study period”
“the DDS data do not show any recent decrease in autism in California despite the exclusion of more than trace levels of thimerosal from nearly all childhood vaccines [my emphasis]. The DDS data do not support the hypothesis that exposure to thimerosal during childhood is a primary cause of autism [my emphasis].
Also in the January Archives of General Psychiatry is an essay, Thimerosal Disappears but Autism Remains, by Eric Fombonne, of the Department of Psychiatry, Montreal Children’s Hospital. Noting that “multiple unfounded theories of causation and corollary ‘treatments’” have arisen since autism was first described in the 1940s, Fombonne writes that “hypotheses on autism-immunization links” have had a “profound impact in the field of autism research and practice and on public health at large.” Indeed: Books claiming a link between vaccines or something in vaccines and autism—journalist David Kirby’s 2005 Evidence of Harm and Jenny McCarthy‘s 2007 Louder Than Words, to name a few—-have been bestsellers and a number of parent-founded advocacy groups have persistently lobbied in support of research about an alleged vaccine-autism link. Parents of young children routinely express fears about vaccinations; parents of autistic children seek, try, and pay for unproven alternative “treatments” based on theories about autism as “mercury poisoning.” As Fombonne writes, these therapies—including chelation, hyperbaric oxygen therapy, and testosterone suppression—”are of unproven efficacy, and many are dangerous.”
Schechter and Grether begin their article by noting that (1) the prevalence rate of autism diagnoses has increased in recent decades; (2) “young children receive immunizations in the period preceding the typical manifestations or diagnosis of ASD”; and (3) increased exposure to thimerosal has been “postulated to have contributed to the upswing in reported cases of ASD.” In noting (3), Schechter and Grether cite two articles that appeared in Medical Hypotheses, a 2001 article including Sallie Bernard and Lyn Redwood as co-authors, and a 2004 article listing Bernard, Redwood, and Mark Blaxill as co-authors; all three are also members of the Executive Board of Safe Minds, which claims that there is a link between the “dramatic rise in autism rates” and mercury in the form of thimerosal.
Schechter and Grether review the use of thimerosal as a preservative in vaccines. It was used to precent “microbial contamination of vaccines, especially those in multdiose vials, since the 1930s.” In 1997, they write:
“prior to any hypothesis that thimerosal might cause autism, the US Food and Drug Administration, in response to its Modernization Act of 1997, compiled and analyzed a list of vaccines and their thimerosal content. By 1999, it became recognized that, under the recommended childhood immunization schedule, infants at 6 months of age were potentially exposed to cumulative doses of ethylmercury that, using an inexact surrogate benchmark in the absence of data, exceeded safety standards (maximum values of which vary from 65-501 µg) for ingestion of another mercury compound, methylmercury. In July 1999, the American Academy of Pediatrics and the US Public Health Service agreed as a precautionary measure that thimerosal be removed as soon as possible from childhood vaccines while maintaining high vaccination coverage levels of children. By 2000, new lots of all Hib and hepatitis B virus vaccines in the United States contained at most trace amounts of thimerosal. By March 2001, all vaccines in the recommended infant immunization schedule for the United States became available with at most trace amounts of thimerosal; the remaining lots of TCVs [thimerasol-containing vaccine] had expiration dates in 2002. [my emphasis]
The authors reference a number of studies (including the Medical Hypotheses previously noted) that have been published since 1999 that have sought to examine whether exposure to thimerosal in vaccines is associated with autism. Beginning in 2001, the Immunization Safety Review Committee of the Institute of Medicine (IOM) of the National Academies concluded in 2003 that evidence “‘favored rejection of a causal relationship between TCVs and autism,’” following a review of epidemiologic data; it was also noted that “‘if rates of ASD continue to increase following the removal of thimerosal, however, then TCVs could not be the primary cause.’”
And an increase in rates of ASD for children born from 1989 through 2003, when exposure to thimerosal has “decreased to its lowest levels in decades,” is what Schechter and Grether have found. Further, since 2004, DDS clients aged 3 – 5 with autism were higher than DDS clients of the same age with “any eligible condition, including autism.” Two reports (a 2001 report by Blaxill and 2006 report by Mark and David Geier) that interpreted the DDS data as supporting thimerosal as a cause of autism are discussed and their limitations: Blaxill’s 2001 report “reflected incomplete ascertainment because children born in more recent years had had a briefer opportunity to e reported compared with children born in earlier years”; the Geiers 2006 study is described as “fundamentally limited because it evaluated the trend in quarterly increases in DDS autism clients” without taking into account the “age of all or new clients or of the population denominator.” In their study, Schechter and Grether specifically measured the prevalence rate according to the birth year or the age of autistic children enrolled in the DDS.
Schechter and Grether further note the limitations of the DDS database: It was designed not to measure the “occurrence of developmental disabilities in the population,” but to keep track of clients’ data. The database does not include all autistic children in California, but only those autistic children “served by the DDS.” And, the resarchers note the lack of “actual data on the maximum or average thimerosal exposure in California for comparison with trends in autism”; a child can be exposed, for instance, be exposed to thimerosal in utero if his or her mother is exposed to vaccines or Rho(D) immune globulin. As Schechter and Grether note, “these limitations and strengths are shared by other analyses of time trends” that use the DDS data—including studies that have concluded that there is a link between thimerosal and autism. As Fombonne writes in his commentary in the Archives of General Psychiatry, the results of Schechter’s and Grether’s study are all the more significant because the DDS data has been “systematically used by proponents of the thimerasol hypothesis to argue that the rising number of children accessing these services—or the “epidemic” of autism—was linked to the increasing exposure to ethylmercury of US children occurring in the 1990s through the changes in the immunization schedule.” Schechter and Grether provide a “clear and unambiguous test” that shows that the decline in autism rates predicted by proponents of the thimerosal hypothesis did not occur.
Just to repeat: Exposure to thimerosal during childhood is not a primary cause of autism.