Linked to Autism-Like Damage in Mice
Institute of Medicine Says Study Does Not Support Link
Between Mercury, Autism in Humans.
by Laurie Barclay, WebMD Medical News, June 09, 2004
For more articles like this
For the past two
decades, a fierce battle has been raging over whether childhood
vaccines can cause autism. On May 18, many thought the battle
was finally over, laid to rest by a report released by the
Institute of Medicine (IOM) Immunization Safety Review
In the report, the committee says that scientific evidence --
based on a significant number of studies conducted since 2001--
supports no association between autism and either
measles-mumps-rubella (MMR) vaccines or those containing
thimerosal, a mercury-based preservative.
"We believe that these conclusions were rendered prematurely,"
Mady Hornig, MD, lead researcher of a new study of thimerosal,
tells WebMD. She is an associate professor of epidemiology and
director of translational research at the Jerome L. and Dawn
Greene Infectious Disease Laboratory, Mailman School of Public
Health, at Columbia University in New York.
Brain Damage Seen in Mice
Her study showed that giving thimerosal to susceptible mice
caused behaviors and brain changes similar to those found in
autism. This study was one of several reviewed by the IOM
committee before they drafted their statement on vaccine safety.
The study is published in the July issue of Molecular
"This type of study, while certainly interesting, in no way
substitutes for actual human evidence," IOM panelist Steven
Goodman, MD, PhD, MHS, associate professor of oncology and
epidemiology at the Johns Hopkins School of Medicine in
Baltimore, tells WebMD.
"We don't have an animal model for autism and we don't
understand exactly what causes autism. ... So we don't
understand it completely in either system at the moment, and we
certainly don't understand to what extent one is a model for the
Although autism was once considered a rare condition, the number
of children diagnosed with autism has increased tenfold since
1985, to about one in every 1,000 children. Whether this is an
actual increase in the disease or just an increase in the
diagnosis of the condition is not clear.
About one third of children with autism have increased risk of
autoimmune diseases, suggesting that genetics may play a role in
autism. To examine the relationship between genetics and
environment, researchers in the mouse experiment gave thimerosal
to a genetic strain of mice susceptible to autoimmune diseases.
The exposed mice had a limited range of behaviors, reacted
strangely to new environments, were less likely to explore, and
appeared anxious. Their brains were larger, and there were
structural changes in the same brain areas involved in human
Hornig says her findings show that the same genes involved in
predicting mercury-related and immune system damage also predict
[brain] damage. Her findings, she says, show an association with
the development of features reminiscent of autism.
Although the IOM panel considered results from Hornig's
experiment when they drafted its report, the panel put greater
weight on large public health studies in humans, panel
chairwoman Marie C. McCormick MD, ScD, tells WebMD.
An Unlikely Link?
The IOM panel acknowledged that mercury can damage nerves and
the brain and that autism is a devastating disorder meriting
additional research into its causes and treatments. However,
they concluded that pursuing the link between the two is
unlikely to be productive, in part because theories of how the
MMR vaccine and thimerosal could trigger autism lack supporting
"We didn't say that investigations shouldn't continue in the lab
on the effects of mercury, on the effects of thimerosal, and on
the causes and profiles of autism," Goodman says. "Where the
committee thought that research dollars probably shouldn't go,
at least for the moment, are these large-scale [public health]
studies linking autism and thimerosal exposure."
But Hornig counters that the reviewed studies may have been of
poor quality, inadequately estimating the risk associated with
these preservatives. She says they failed to account for
genetics, age, sex, nutrition, dosing schedule, and other
factors known to affect how mercury interacts with the body.
"The pronouncement that research funds are better applied
elsewhere effectively forecloses any possibility of federal
funding for an entire field of research," she says. "The timing
is particularly unfortunate given that we are only just
beginning to define the mechanisms by which environmental
factors such as thimerosal interact with immune response genes
during early development."
McCormick says the IOM committee recommended that "available
funding for autism research be channeled to the most promising
areas, of which the link with vaccines does not appear to be
However, the committee did recommend keeping track of new cases
of autism, which would be expected to decrease along with
declining use of thimerosal-containing vaccines if indeed
mercury increases the risk of autism.
"The IOM crossed a line when they went so far as to say that
there should be no new research on the link between vaccines and
autism," Barbara Loe Fisher tells WebMD. She is president and
founder of the National Vaccine Information Center (NVIC), a
nonprofit organization dedicated to the prevention of vaccine
injuries and deaths through public education.
"In science and in medicine, there are many instances where what
is true today may not be true tomorrow. This report tried to
close the door, and I thought it went too far."
Compared with earlier IOM reports in 1991, 1994, and 2001, which
said that evidence was not sufficient to accept or reject a link
between vaccines and autism, Fisher says the present report uses
stronger language and "too much reliance on [public health]
studies, basically just looking at old medical records, and not
on basic sciences."
Since 2001, five large public health studies conducted in the
U.S., the U.K., Denmark, and Sweden showed evidence that there
is no association between thimerosal-containing vaccines and
The use of thimerosal in vaccines has declined since
manufacturers began removing it from childhood vaccines in 1999.
Since 2001, all universally recommended childhood vaccines are
available in single-dose vials containing no thimerosal. Certain
flu vaccines and tetanus-diphtheria vaccines, however, given to
children age 7 and older contain thimerosal as a preservative.
The MMR vaccine has never contained thimerosal.
"It was clear from the report that we were not giving thimerosal
a clean bill of health," Goodman says.
"We didn't say that thimerosal is something that we should want
in vaccines; we said that the safest vaccines are indeed
thimerosal-free vaccines. We only said that the evidence favored
that there was not a connection between autism and thimerosal
An additional concern regarding mercury exposure from thimerosal
is how it might add together with other mercury exposures, such
as eating seafood. Hornig says that ethylmercury, the form found
in thimerosal, causes more damage to the developing brain than
does methylmercury, the form that is found in fish.
"We live in a world where there is exposure to mercury from
multiple sources, and some of it may be unavoidable," Goodman
says. "To the extent that there is mercury in the air, we can't
stop breathing. To the extent that there is mercury in certain
foods, obviously we can avoid fish, but anything else we may not
The IOM panel recommended further studies of the amount of
mercury exposure from thimerosal and other forms of mercury in
infants, children, and pregnant women.
"Thimerosal was removed from vaccines as a precautionary measure
to reduce the exposure to mercury from all sources," McCormick
says. "There is no evidence that the amount of mercury present
in vaccines resulted in health problems."
The IOM committee emphasized that children should still receive
the full course of recommended vaccinations to prevent
infectious diseases and that they should get thimerosal-free
vaccines to minimize overall mercury exposure. These
recommendations are consistent with those of the American
Academy of Pediatrics and the U.S. Public Health Service, who in
July 1999 issued a joint statement calling for the removal of
thimerosal from vaccines.
Thimerosal Still Around?
On May 20, the Office of Special Counsel, an independent
investigative and prosecutorial agency handling "whistleblower"
complaints, forwarded to congressional oversight committees
hundreds of disclosures alleging public health and safety
concerns regarding the possible link between
thimerosal-containing vaccines and autism, many from parents of
children with autism or other neurological disorders.
Contrary to statements from Department of Health and Human
Services agencies, the HHS Office of Investigations, and the
American Academy of Pediatrics, the disclosures claim that some
childhood vaccines with expiration dates of 2005 contain 25
micrograms of mercury and continue to be produced and
As a next step, the IOM panel recommends developing programs to
increase public participation in vaccine safety research and
policy decisions and to promote constructive dialogue between
scientists, government officials, and the public about research
findings and their implications for policy development. For the
past two decades, the NVIC has been mandating independent,
non-governmental, non-industry research into the genetic and
other biological high risk factors of vaccine-associated brain
and immune system dysfunction, including autism.
"There needs to be more transparency, access to the data by more
researchers, and a greater collaborative effort by the
government or there will be a loss of trust," Fisher says. "I am
cautiously hopeful that they will do this."
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