the MMR Vaccine Cause Autism?
By Jeffrey P. Baker, MD, PhD and Dennis Clements, MD, PhD
one disputes that autism is being diagnosed more frequently than
it was twenty years ago. But experts debate to what extent this
trend reflects better detection or a real increase in the disorder
itself. The definition of autism has been expanded considerably
in recent years, and better recognition likely accounts for much
of the rise. Indeed, studies of families and twins suggest that
genetic factors are of primary importance.
for parents coping with the demands of an autistic child, it is
hard not to believe that something in the environment is at fault
as well. Many possible causes have been proposed, ranging from food
additives to environmental mercury and PCBs. The National Autism
Association has focused on the vaccine preservative thimerosal,
metabolized to ethyl mercury, as a possible culprit. There is simply
no evidence supporting this assertion, however. Denmark removed
thimerosal from its vaccines in 1992 and still experienced a subsequent
rise in autism. The United States has done the same, and today all
of the required childhood vaccines are now available without this
preservative. Again, no association with autism has been shown.
of the MMR vaccine
The other chief vaccine controversy involves the MMR immunization,
which protects against measles, mumps, and rubella (German measles).
First licensed in the United States in 1971, it long had a reputation
as one of the safest and most effective vaccines.
The most serious of the three diseases against which it protects,
measles, was once a nearly universal affliction in childhood. At
best it involves several miserable days of high fever followed by
a rash. What makes it a greater concern, however, are its complications
of pneumonia and encephalitis (an infection that can permanently
injure the brain). In the last major epidemic in the United States
(1990-1991), 20 percent of children who caught the disease were
hospitalized and 1 in 400 died. The dangers of measles have largely
been forgotten since widespread use of the vaccine has resulted
in a 99 percent decline in the incidence of the disease in this
charges against MMR
In 1997, however, British investigator Andrew Wakefield published
an article in which he described several children who developed
signs of autism and intestinal symptoms following their MMR immunization.
He suggested that the vaccine inflamed the gut in a manner that
allowed an unspecified toxic substance to cross into the bloodstream.
As the British media picked up the story, anecdotes from parents
began to circulate in support.
Out of this modest study has arisen a formidable challenge to the
MMR vaccine. Vaccination rates in Britain have dropped to the point
that outbreaks of the disease are again occurring. Many parents
of autistic children on both sides of the Atlantic have embraced
many parents are just not sure what to think. Is the MMR vaccine
really guilty as charged?
to autism: A matter of timing?
The fundamental question is whether the relationship of the vaccine
to autism is real or coincidental. The MMR is generally given to
infants 12 to 15 months old. Children with autism are often described
by the parents as developing normally until regressing (or losing
language and social abilities) during the second year of life. Many
autism researchers would not agree that most autistic children progressed
truly "normally" during the first year, but it remains
true that some autistic children do regress dramatically as toddlers.
And in certain cases this will inevitably occur soon after the MMR
vaccine. The question is whether these distressing cases represent
a cause-and-effect relationship or a tragic coincidence.
This question has been intensively examined by both large studies
and expert panel reviews. Panels convened by the Institute of Medicine,
Medical Research Council, and World Health Organization have all
agreed that these studies have not supported the hypothesis that
MMR is an important cause of autism.
parents cannot help but wonder whether the experts are right. Statistics
are complex and subject to manipulation. A host of Internet sites
charge that this is exactly what has happened.
the worry arose
Here is where understanding a bit of medical history can be of help.
It is no accident that the MMR/autism controversy resonated with
so many parents in Britain. Autism cases began to rise roughly around
1980. Public talk about an "autism epidemic" gained momentum
in the 1990s. And in Britain, the MMR was a relatively new vaccine,
introduced into the routine child vaccine schedule only in 1988.
In the United States, as previously mentioned, MMR has been used
since 1971. Measles vaccine was licensed still earlier (in 1963).
Yet there was no talk of an autism epidemic then.
The MMR/autism controversy, in other words, is a British import
to the U.S. In this regard it is worth noting that such controversies
have been a recurrent theme in the United Kingdom since the bitter
controversies surrounding compulsory smallpox vaccination in the
19th century. In 1978 and 1982, whooping cough epidemics swept the
British Isles in the wake of widespread public fear that its vaccine
carried a significant risk of brain damage. That alleged association
was shown at best to be extremely rare if based in reality at all.
find no link between MMR and autism
If MMR caused a significant fraction of the rise in cases of autism,
one would expect that its introduction would have been associated
with a jump in autism cases. Studies have examined this question
in both California and Britain. In neither case has any link been
Far from being covered up, the MMR/autism hypothesis has been intensively
studied. One of the authors of Wakefield's original paper has recently
stepped forward to say that "There is now unequivocal evidence
that MMR is not a risk factor for autism -- this statement is not
spin or medical conspiracy, but reflects an unprecedented volume
of medical study."
parents of autistic children are some of the most courageous and
dedicated in my practice. I can only begin to grasp their level
of frustration surrounding the many uncertainties that attend the
cause of autism. The studies admittedly cannot disprove that MMR
has ever provoked a single case of autism. But they do show, unequivocally,
that if this has happened at all, it has happened only extremely
rarely, and cannot begin to account for the rise in autistic cases
that has caused so much distress. There is no doubt, on the other
hand, that the dangers from measles itself are real and well-documented.
P. Baker, MD, PhD, is an associate clinical professor of pediatrics
and on the faculty of the Duke Center for the Study of Medical Ethics
and Humanities. Dennis
Clements, MD, PhD, is interim chair of the Department of Pediatrics
at Duke University Medical Center. For more information, visit www.dukehealth.org