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Do vaccines cause autism? |
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Get the
facts about vaccines and autism |
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One of the main
concerns of parents regarding vaccination of their infants is that
vaccines are safe. A frequent “scare” about vaccines is whether they
cause disorders of the brain or nervous system. The most common
scare mentioned by the news media and anti-vaccination groups is
that vaccines cause pervasive developmental delay in children, more
commonly called
autism. Numerous large research studies have shown
that this is not the case. However, it is easy to see how this
misunderstanding arose.
Autism is
usually diagnosed in the first three years of life, most commonly
between the ages of one and two years, a time that coincides with
completion of the primary vaccination series. More cases of autism
are being diagnosed now compared to 20 years ago, although some of
this increase may be due to improved detection of previously missed
“mild cases” or because of changes in the definition of autism.
Finally, as the cause of autism is not known, events that occur by
chance at the same time as the diagnosis sometimes are linked in
parents’ minds as possible associations.
Read about the seven key signs of autism
No single factor has been identified as the cause of autism.
However, home movies taken in the first year of life suggest that
subtle signs of the disorder can be evident as early as two to three
months of age (before vaccination begins) suggesting that the
underlying cause is present before birth. There is evidence that
multiple factors may be involved in causing the condition. These
include:
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Genetics
There is considerable evidence that genes have a strong
influence on the development of autism. The best evidence comes
from studies of autism in twins. Investigators have shown that
when one identical twin has autism, there is at least a 90
percent chance that the other twin (who has exactly the same
genes) also will have autism. However, a fraternal twin, who
does not share the same genes with the other twin, but who
presumably shares other potential risk factors for autism both
before and after birth, has only a 10 percent chance of
developing autism if the other twin is affected.
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Insults
to the fetus before birth
Studies have also shown there is a vulnerable time early in
pregnancy when toxic substances or infections can cause autism.
For example, thalidomide was taken by numerous pregnant women
during the 1960s to control morning sickness. Thalidomide was
later discovered to cause significant limb and ear abnormalities
in fetuses. These infants also were much more likely to develop
autism than infants whose mothers never took thalidomide. This
increased risk occurred only if thalidomide was taken in the
early stages of pregnancy.
Similarly, before rubella vaccination
was introduced in the United States, women who developed rubella
early in pregnancy were at risk of having babies with congenital
rubella syndrome (CRS), a serious disorder involving eye, ear,
heart and neurological birth defects. Babies with CRS also have
a higher risk of developing autism. However, babies who develop
rubella infection soon after birth are not at higher risk of
autism. These two facts suggest that factors causing autism
occur early in pregnancy rather than near the time of delivery
or after birth.
Concerns that a vaccine preservative could cause autism also
received much media attention within the past decade. Preservatives
in vaccines are necessary to prevent their contamination by bacteria
and fungi. Questions were raised about the safety of this practice
because the chosen preservative was thimerosal, which contains
mercury.
Mercury is a
naturally occurring element that is changed into methylmercury by
bacteria in the environment. Methylmercury can then be taken into
the body through the food chain (for example, in fish and animal
meats). Agencies such as the
Environmental Protection Agency (EPA),
Food and Drug Administration Agency (FDA) and
World Health Organization (WHO) have established guidelines for the maximum
amount of methylmercury to which humans should be exposed. These
levels are many times less than the levels thought to be toxic.
Because high levels of methylmercury can be toxic to humans, concern
was expressed that the thimerosal present in vaccines could cause
autism. However, this argument does not hold true for the following
reasons:
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The mercury
contained in thimerosal is a different form of mercury (ethylmercury) than the methylmercury that occurs naturally.
Ethylmercury (thimerosal) is less likely to accumulate in the
body and cause harm than methylmercury because it is metabolized
and excreted far quicker than methylmercury.
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The signs of
mercury poisoning have been compared with the symptoms of autism
by comparing head size, speech, vision, sensation and
coordination among children with each condition. There is a
significant difference in the symptoms of these two conditions.
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There is no
evidence that mercury causes autism and there is a lot of evidence
demonstrating that it doesn’t. For example, in 1971 Iraq
imported grain that had been fumigated with methylmercury. The
methylmercury entered the food chain through bread made with
this grain and resulted in one of the worst outbreaks of mercury
poisoning in history. This outbreak caused 6,500 hospitalizations
and 450 deaths. Pregnant women who ate the bread delivered
babies who had epilepsy and mental retardation. However, their
babies were not more likely to develop autism.
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Large
studies have been performed in the United States, United
Kingdom, Sweden and Denmark comparing the risk of autism in
children who received vaccines that contained thimerosal with
those who received vaccines that did not contain thimerosal. All
studies showed the same thing: there was no increased risk of
autism in children who received vaccines containing thimerosal.
As a matter of fact, the incidence of autism actually increased
in Denmark the years after thimerosal was removed from vaccines.
-
The
naturally occurring methylmercury is present in water,
infant formula and breastmilk. A breastfed infant will receive
more than twice the amount of mercury from breastmilk than was
contained in vaccines.
The
Institute of
Medicine rejected “a causal relationship between thimerosal vaccines
and autism” in 2004. However, because a
formula-fed infant who received multiple vaccines on a single day
might potentially
be exposed to a level of mercury higher than the EPA guideline (but
falling below the FDA or WHO guidelines), it was decided where
possible to remove thimerosal from all vaccines routinely given to
infants and children (except the
inactivated influenza
vaccine) in the United States. As explained above, this is not
because it was shown to cause autism. It was because alternative
preservatives are available and because the risk of
contamination is less now that single dose vials of vaccine are used
rather than multiple dose vials.
There has also been considerable interest in the theory that the MMR
vaccine causes autism. Vaccines given at 15 to18 months of age are
particularly likely to be associated in parents’ minds with the
development of autism because it is often after this age that signs
become obvious. This fear was exaggerated in 1998 after the journal
Lancet published the Wakefield study of 12 children (eight
with autism) who had intestinal complaints and developed autism
within one month of receiving MMR vaccine. The authors suspected
that MMR caused intestinal inflammation, which than allowed proteins
to enter the bloodstream and pass to the brain causing autism. However,
this theory was flawed because in all eight patients with autism the
signs of autism occurred before the symptoms of intestinal
inflammation.
To
prove a causal relationship, the authors should have studied the
incidence of autism in unvaccinated children, and this was not done. A
follow-up paper published in 2002 by Wakefield and colleagues also was flawed for this reason. Ten of the authors of the original 1998
paper, the hospital the patients attended and the Lancet have disassociated themselves from the original study suggesting that MMR caused autism.
Large, well-controlled studies have been performed in the United
States, United Kingdom and Denmark over long periods of time to
examine the incidence of autism in children who did and did not
receive the MMR vaccine. The incidence of autism did not increase in
vaccinated children in any of these studies. The Institute of
Medicine in 2004 rejected “a causal relationship between the MMR
vaccine and autism."
However, the
damage done by the MMR “scare” in 1998 was sufficient to cause
vaccine rates to fall, which then led to outbreaks of measles, mumps
and rubella in Britain and other countries in Western Europe.
While there is understandable anxiety as to the cause of autism in
children, it is important to remember that the available evidence
points to it being a genetic disorder or caused by some insult to
the fetus early in pregnancy. Signs of autism become more obvious
around the time that infants have completed their primary
vaccination series; however, studies have shown that subtle signs of
the disorder clearly occur before vaccination begins. Fears that thimerosal or MMR vaccine could cause autism have proven unfounded
in repeated large-scale studies in many countries
throughout the world. There is no evidence that vaccines cause
autism.
 
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