Claims Data Fail to Support Thimerosal-Autism Link

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Claims Data Fail to Support Thimerosal-Autism Link

By Gever, Senior Editor, MedPage Today

September 13, 2010

MedPage Today Action Points

* Discuss with parents that data from a recent case control study indicates

no increased risk of autism from prenatal and infant vaccination with

thimerosal-containing vaccines.

* Further discuss with parents that this latest study adds to data collected

from 4 previous studies, all of which fail to demonstrate any connection between

the development of autism and prior exposure to thimerosal-containing vaccines.

Review

Yet another study -- this one analyzing insurance claims data and other records

for children with autism spectrum disorders -- showed that exposure to

thimerosal, either prenatally or after birth, did not increase the risk of

autism, researchers said.

The case-control study, of 256 children with autism spectrum disorders and 752

age- and sex-matched healthy controls, found that higher-than-average exposures

to ethylmercury were, if anything, less common in kids with autism compared with

healthy kids, reported DeStefano, MD, MPH, of the CDC's Immunization

Safety Office in Atlanta, and colleagues.

The study of thimerosal exposures, conducted among children belonging to three

managed care organizations, was published in the journal Pediatrics. It was the

latest of a number of studies that found no link between thimerosal and autism.

The research team found the following odds ratios for autism spectrum disorders

associated with increases in ethylmercury exposure of two standard deviations,

after adjusting for numerous potential confounders such as birthweight and

maternal age:

* Prenatal exposure: OR 1.12 (95% CI 0.83 to 1.51)

* Exposure from birth to one month of age: OR 0.88 (95% CI 0.62 to 1.26)

* Exposure from birth to seven months: OR 0.60 (95% CI 0.36 to 0.99)

* Exposure from birth to 20 months: OR 0.60 (95% CI 0.32 to 0.97)

Noting that increased ethylmercury exposure in the two longer postnatal periods

actually appeared to decrease the risk of three autism spectrum disorder

outcomes, DeStefano and colleagues wrote that they were " not aware of a

biological mechanism that would lead to this result. "

But they were sure of one thing: the data yielded " no evidence that increasing

ethylmercury exposure from thimerosal-containing immunoglobulins was associated

with increased risk of autism spectrum disorders, autistic disorder, or

[spectrum disorders] with regression. "

The analysis is the latest in a long parade of studies, using a variety of

methods and data sources, to find no connection between vaccines containing

thimerosal (once commonly used as a preservative) and autism.

The flap over thimerosal and autism led vaccine manufacturers to remove the

preservative from most of their products by 2001. Yet new autism diagnoses have

soared since then, whereas a decrease in autism would be expected if thimerosal

was an important cause of the condition.

Although the connection is now widely discredited, DeStefano and colleagues had

access to an unusually detailed dataset that could more firmly establish whether

or not such a link could exist.

Not only did the researchers have immunization records for the children, but

they also had direct access to the children for in-person assessments and to

their mothers for interviews to establish prenatal exposures and other

potentially relevant factors.

The participants chosen for analysis were born from 1994 to 1999 and were

continuously enrolled in the three managed care organizations from birth through

24 months of age, as well as currently enrolled when the analysis was conducted.

At that time, the children were six to 13 years old.

Ethylmercury exposures were calculated from computerized immunization records

and review of medical charts. The mercury content of each vaccine dose was

obtained from lot-specific manufacturers' records and published data. Prenatal

exposures were calculated from mothers' receipt of immunoglobulins, tetanus

toxoids, and diphtheria-tetanus immunizations during pregnancy.

Average cumulative ethylmercury exposure for the four periods (prenatal, birth

to one month, birth to seven months, and birth to 20 months) was nearly

identical in every comparison between cases and controls.

Although there was no increase in the risk for autism spectrum disorders as a

group with increased mercury exposure, DeStefano and colleagues also looked at

whether there might be a specific relationship with autistic disorder or

spectrum disorders with regression.

The analysis of 187 children with an autistic disorder and to the 49 with autism

spectrum disorders with regression showed the same patterns as in the larger

group.

In fact, in the latter group, the decreased risk seen with increased

ethylmercury exposure over seven and 20 months after birth was especially

pronounced.

Comparing these 49 children with 652 age- and sex-matched controls produced odds

ratios of 0.21 (95% CI 0.08 to 0.60) for exposures from birth to seven months

and 0.30 (95% CI 0.08 to 0.76) for exposures from birth to 20 months.

" No single study can definitively establish or disprove the hypothesis that

thimerosal exposure increases the risk of autism spectrum disorders, " DeStefano

and colleagues pointed out, given that randomized trials would be unethical.

On the other hand, they noted that a 2004 review by the Institute of Medicine

involving multiple studies concluded that the link between autism and thimerosal

was unsupported -- and four additional studies using different methodologies

also failed to demonstrate any connection.

DeStefano and colleagues did identify several limitations of their own study:

the possibility of unmeasured confounding factors, potential recall and

reporting bias in the maternal interviews, and the uncertainty inherent in

diagnoses of autism spectrum disorders.

The study was supported by a contract from the CDC to America's Health Insurance

Plans and via America's Health Insurance Plans subcontracts to the authors'

institutions.

Study authors reported speaking fees from Merck and GlaxoKline and research

funding from Medimmune, Sanofi Pasteur, Chiron, Wyeth, Merck, and

GlaxoKline.

Primary source: Pediatrics

Source reference:

Price C, et al " Prenatal and infant exposure to thimerosal from vaccines and

immunoglobulins and risk of autism " Pediatrics 2010; 126: 656-64.

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