Vaccine Safety > Issues of Interest >
Autism FAQs (frequently asked questions) about MMR Vaccine & Autism (Measles, Mumps, and
Rubella)
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At
a glance: The weight of currently available
scientific evidence does not support the hypothesis that MMR
vaccine causes autism. CDC recognizes there is considerable
public interest in this issue, and therefore supports
additional research regarding this hypothesis. CDC is
committed to maintaining the safest, most effective vaccine
supply in history. |
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Frequently Asked Questions |
- What is
autism?
Autism is a term that refers to a collection of
neurologically-based developmental disorders in which individuals
have impairments in social interaction and communication skills,
along with a tendency to have repetitive behaviors or interests.
The severity of autism varies greatly, from individuals with
little speech and poor daily living skills, to others who function
well in most settings. Autism is typically diagnosed during the
toddler or preschool years, although some children are diagnosed
at older ages. It has been reported that approximately 20 percent
of children with autism experience a "regression;" that is, they
have apparently normal development followed by a loss of
communication and social skills. Boys are three-to-four times more
likely to have autism than girls. Autism occurs in all racial,
ethnic, and social groups. A variety of factors could be
associated with some forms of autism, including infectious,
metabolic, genetic, neurological, and environmental factors.
Genetic factors and brain abnormalities at birth are considered to
be some of the most recognized causes of autism. For more
information, see CDC's autism
site
(http://www.cdc.gov/ncbddd/dd/ddautism.htm)
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- Does the
measles-mumps-rubella (MMR) vaccine cause autism?
Current scientific evidence does not support the hypothesis
that measles-mumps-rubella (MMR) vaccine, or any combination of
vaccines, causes the development of autism, including regressive
forms of autism. The question about a possible link between MMR
vaccine and autism has been extensively reviewed by independent
groups of experts in the U.S. including the National Academy of Sciences, Institute
of Medicine. These reviews have concluded that the available
epidemiologic evidence does not support a causal link between MMR
vaccine and autism.
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- What have
studies found regarding MMR vaccine and autism?
Epidemiologic studies have shown no relationship between
MMR vaccination in children and development of
autism:
- In
1997, the National Childhood Encephalopathy Study (NCES) was
examined to see if there was any link between measles vaccine and
neurological events. The researchers found no indication that
measles vaccine contributes to the development of long-term
neurological damage, including educational and behavioral deficits
(Miller et al., 1997).
- A
study by Gillberg and Heijbel (1998) examined the prevalence of
autism in children born in Sweden from 1975-1984. There was no
difference in the prevalence of autism among children born before
the introduction of the MMR vaccine in Sweden and those born after
the vaccine was introduced.
- In
1999, the British Committee on Safety of Medicines convened a
"Working Party on MMR Vaccine" to conduct a systematic review of
reports of autism, gastrointestinal disease, and similar disorders
after receipt of MMR or measles/rubella vaccine. It was concluded
that the available information did not support the posited
associations between MMR and autism and other
disorders.
- Taylor and colleagues (1999) studied 498 children with
autism in the UK and found the age at which they were diagnosed
was the same regardless of whether they received the MMR vaccine
before or after 18 months of age or whether they were never
vaccinated. Importantly, the first signs or diagnoses of autism
were not more likely to occur within time periods following MMR
vaccination than during other time periods. Also, there was no
sudden increase in cases of autism after the introduction of MMR
vaccine in the UK. Such a jump would have been expected if MMR
vaccine was causing a substantial increase in
autism.
- Kaye
and colleagues (2001) assessed the relationship between the risk
of autism among children in the UK and MMR vaccine. Among a
subgroup of boys aged 2-5 years, the risk of autism increased
almost 4 fold from 1988 to 1993, while MMR vaccination coverage
remained constant at approximately 95% over these same
years.
- Researchers in the U.S. found that among children born
between 1980 and 1994 and enrolled in California kindergartens,
there was a 373% relative increase in autism cases, though the
relative increase in MMR vaccine coverage by the age of 24 months
was only 14% (Dales et al., 2001). For more on this study, see California
Data on Theory of Autism and MMR
Immunization.
- Researchers in the UK (Frombonne & Chakrabarti, 2001)
conducted a study to test the idea that a new form, or "new
variant," of Inflammatory Bowel Disease (IBD) exists. This new
variant IBD has been described as a combination of developmental
regression and gastrointestinal symptoms occurring shortly after
MMR immunization. Information on 96 children (95 immunized with
MMR) who were born between 1992 and 1995 and were diagnosed with
pervasive developmental disorder were compared with data from 2
groups of autistic patients (one group of 98 born before MMR was
ever used and one group of 68 who were likely to have received MMR
vaccine). No evidence was found to support a new syndrome of
MMR-induced IBD/autism. For instance, the researchers found that
there were no differences between vaccinated and unvaccinated
groups with regard to when their parents first became concerned
about their child’s development. Similarly, the rate of
developmental regression reported in the vaccinated and
unvaccinated groups was not different; therefore, there was no
suggestion that developmental regression had increased in
frequency since MMR was introduced. Of the 96 children in the
first group, no inflammatory bowel disorder was reported.
Furthermore, there was no association found between developmental
regression and gastrointestinal symptoms.
- Another group of researchers in the UK (Taylor et al.,
2002) also examined whether MMR vaccination is associated with
bowel problems and developmental regression in children with
autism, looking for evidence of a "new variant" form of
IBD/autism. The study included 278 cases of children with autism
and 195 with atypical autism (cases with many of the features of
childhood autism but not quite meeting the required criteria for
that diagnosis, or with atypical features such as onset of
symptoms after the age of 3 years). The cases included in this
study were born between 1979 and 1998. The proportion of children
with developmental regression or bowel symptoms did not change
significantly from 1979 to 1988, a period which included the
introduction of MMR vaccination in the UK in 1988. No significant
difference was found in rates of bowel problems or regression in
children who received the MMR vaccine before their parents became
concerned about their development, compared with those who
received it only after such concern and those who had not received
the MMR vaccine. The findings provide no support for an MMR
associated "new variant" form of autism and further evidence
against involvement of MMR vaccine in
autism.
- Madsen et al. (2002) conducted a study of all children born
in Denmark from January 1991 through December 1998. There were a
total of 537,303 children in the study; 440,655 of the children
were vaccinated with MMR and 96,648 were not. The researchers did
not find a higher risk of autism in the vaccinated than in the
unvaccinated group of children. Furthermore, there was no
association between the age at time of vaccination, the amount of
time that had passed since vaccination, or the date of vaccination
and the development of any autistic disorder. Though there were
many more vaccinated than unvaccinated children in the study
group, the sample was large enough to contain more statistical
power than other MMR and autism studies. Therefore, this study
provides strong evidence against the hypothesis that MMR
vaccination causes autism.
- DeStefano et al. (2004) conducted a study to see if there
was a difference in the age at which children with autism and
without autism received their first MMR vaccination. The study's
findings showed that children with autism received their first MMR
vaccination at similar ages as children without autism. More
information about this study can be found on the CDC's
research on vaccines and autism web page.
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- Are there studies that suggest there might be
a connection between autism and MMR vaccine?
The
existing studies that suggest a causal relationship between MMR
vaccine and autism have generated media attention. However, these
studies have significant weaknesses and are far outweighed by the
epidemiologic studies described above that have consistently
failed to show a causal relationship between MMR vaccine and
autism.
- The
MMR-autism theory is based on the idea that intestinal problems,
like Crohn’s disease, are the result of viral infection and can
contribute to the development of autism. The theory has its
origins in research by Wakefield and colleagues (1989; 1990) which
suggested that inflammatory
bowel disease (IBD) is linked to persistent viral
infection.
- In
1993, Wakefield and colleagues reported isolating measles virus in
the intestinal tissue of persons with IBD. However, the validity
of this finding was later called into question when it could not
be reproduced by other researchers (Afzal, 1998; Iizuka et al.,
2000).
- Thompson and colleagues (1995) suggested in a retrospective
cohort study that MMR vaccine might be a risk factor for Crohn's
disease. However, the selection and recall biases and the
differences in data collection in this study were so substantial
as to cast doubt on the validity of the
findings.
- Two
studies out of Sweden linked measles infection in utero to the
development of IBD (Ekbom et al., 1994; Ekbom et al., 1996).
However, these studies involved a very small number of cases and
when researchers identified the persons to be included in the 1996
study, they had prior knowledge that cases of Crohn’s disease had
occurred in the offspring of two women who were infected with
measles during pregnancy. This is called "selection bias" and
limits the strength of the study.
- The
MMR-autism theory came to the forefront when, in 1998, Wakefield
and colleagues reviewed reports of children with bowel disease and
regressive developmental disorders, mostly autism. The researchers
suggested that MMR vaccination led to intestinal abnormalities,
resulting in impaired intestinal function and developmental
regression within 24 hours to a few weeks of vaccination. This
hypothesis was based on 12 children. In 9 of the cases, the
child's parents or pediatrician speculated that the MMR vaccine
had contributed to the behavioral problems of the children in the
study. There are a number of limitations in the Wakefield et al.
(1998) study:
- The study used too few cases to make any generalizations
about the causes of autism; only 12 children were included in
the study. Further, the cases were referred to the researchers
and may not be a representative sample of cases of
autism.
- There were no healthy control children for comparison. As
a result, it is difficult to determine whether the bowel changes
seen in the 12 children included in the study were similar to
changes in normal children, or to determine if the rate of
vaccination in autistic children was higher than in the general
population.
- The study did not identify the time period during which
the cases were identified.
- In
at least 4 of the 12 cases, behavioral problems appeared
before the onset of symptoms of bowel disease; that is,
the effect preceded the proposed cause. It is unlikely,
therefore, that bowel disease or the MMR vaccine triggered the
autism.
In
2004, 10 of the 13 authors of the study retracted the paper's
interpretation, stating that the data were insufficient to
establish a causal link between MMR vaccine and autism (Murch et
al., 2004)
- In
another study that generated media attention and raised public
concern in the UK (Uhlmann et al, 2002), researchers found measles
virus fragments in the intestines of children with "new variant"
IBD (children with both IBD and developmental disorder).
Scientists looked for the presence of measles virus in the
intestinal tissue of 91 children with new variant IBD and 70
"controls" (children without this type of IBD). The researchers
found measles virus fragments in 75 out of the 91 children with
"new variant" IBD, and in only 5 of the 70 controls. While this
provides evidence for an association between the presence
of measles virus and IBD in children with developmental disorder,
it does not mean that the measles component of the MMR vaccine
causes IBD or developmental disorder. As a commentary
published with the article asserts, the data could just as easily
be interpreted as indicating that the IBD or the developmental
disorder cause the persistence of measles in the intestines
(Morris & Aldulaimi, 2002). In addition, the researchers did
not compare the virus found in the intestines of patients with the
virus used in the MMR vaccine; nor did they provide information
regarding whether or not the children in the study had been
previously vaccinated with MMR or had previously contracted
measles disease. The limitations of this study are further
discussed in a letter written by the Director of CDC’s National
Immunization Program to the UK’s Chief Medical Officer.
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- What about the claim that the
number of children with autism has been increasing ever since the
MMR vaccine has been in use?
Data
from California (Department. of Developmental Services, 1999) have
been used to illustrate an increase in cases of autism since the
introduction of MMR vaccine. However, the data have been presented
inaccurately (Fombonne, 2001). Fombonne (2001) lists several
reasons why the data are misrepresented, for
instance:
- the figures presented are based on numbers, not rates and
do not account for population growth and changes in the
composition of the population,
- changes in diagnostic definitions were not controlled in
the report, and
- as
in other areas of the country, children with autism are
currently being diagnosed at earlier ages meaning that there
will be an increase in the number of reported cases.
A 2001
study (Dales et al.) used the autism case numbers provided by
the California Department of Developmental Services and compared
them with early childhood MMR immunization level estimates for
California children. Results showed that for children born from
1980 through 1987, there was no major change in MMR immunization
levels with the exception of a small increase in children born in
1988. This small increase was followed again by steady levels in
children born through 1994. On the other hand, the cases of autism
increased markedly, from 44 cases per 100,000 live births in 1980
to 208 cases per 100,000 live births in 1994. Even if one allows
that a true increase in autism has occurred and the increase is
not due to changes in diagnostic methods, diagnostic
categorization, and improved identification of individuals with
autism because of the level of services offered (Fombonne, 2001),
this analysis shows that receipt of the MMR vaccine is not a
factor. If it were a factor, one would expect the shape of the MMR
level of immunization curve to be very similar to the autism case
numbers. This is not the case, thus the analysis in this study
argues against a link between MMR vaccination and
autism.
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- Would it be
safer to separate the MMR vaccine into its individual
components--in other words, give children three separate shots, at
different times (e.g., six months or one year apart), instead of
one combined shot?
There
is no confirmed scientific research or data to indicate that there
is any benefit to separating the MMR vaccine into its individual
components. A publication by Wakefield and Montgomery
(2001) suggests that there is an increased risk of immune-mediated
disease when the MMR vaccine is administered as one vaccine versus
when the 3 vaccines are administered separately. The specific
issue of the safety of multiple vaccines given as one vaccine was
addressed by the Institute of Medicine (IOM) (1994, p.63). They
stated that the number of separate antigens in a vaccine would not
likely result in a significant burden on the immune system that
would result in immunosuppression. The issue of multiple
vaccines and immune dysfunction was addressed again by the IOM
in 2002. An IOM Immunization Safety Review Committee concluded
that a review of the available scientific evidence does not
support the suggestion that the infant immune system is inherently
incapable of handling the number of antigens that children are
exposed to during routine immunizations. The IOM committee also
did not suggest any need to change the current US vaccination
schedule for MMR.
Splitting the MMR vaccine into three separate doses given
at three different times would cause more discomfort from
additional injections and would leave children exposed to
potentially serious diseases. For instance, if rubella vaccine
were delayed, 4 million children would be susceptible to rubella
for an additional 6 to 12 months. This would potentially allow
otherwise preventable cases of congenital rubella syndrome (CRS)
to occur through transmission of rubella from infected children to
pregnant women. Ironically, infection of pregnant woman with
"wild" rubella virus is one of the few known causes of autism.
Thus, by preventing rubella infection of pregnant women, MMR
vaccine also prevents autism.
Chess,S. Autism in children
with congenital rubella. J Autism Child Schizophr. 1, 33-47
(1971).
Chess,S. Follow-up report on autism in congenital rubella.
J Autism Child Schizophr. 7, 69-81 (1977).
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- Should a
younger sibling of an autistic child, or a child of someone who
has autism be vaccinated with MMR or other
vaccines?
Yes.
Current scientific evidence does not show that MMR vaccine, or any
combination of vaccines, causes the development of autism,
including regressive forms of autism.
A
younger sibling or the child of someone who suffered a vaccine
side effect usually can, and should, safely receive the same
vaccine. This is especially true since the large majority of side
effects after vaccination are local reactions and fever, which do
not represent a contraindication.
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- Should we
delay vaccination until we know more about the negative effects of
vaccines?
No.
There is no convincing evidence that vaccines such as MMR cause
long term health effects. On the other hand, we do know that
people will become ill and some will die from the diseases this
vaccine prevents. Measles outbreaks have recently occurred in the
UK and Germany following an increase in the number of parents who
chose not to have their children vaccinated with the MMR vaccine.
Discontinuing a vaccine program based on unproven theories would
not be in anyone's best interest. Isolated reports about these
vaccines causing long term health problems may sound alarming at
first. However, careful review of the science reveals that these
reports are isolated and not confirmed by scientifically sound
research. Detailed medical reviews of health effects reported
after receipt of vaccines have often proven to be unrelated to
vaccines, but rather have been related to other health factors.
Because these vaccines are recommended widely to protect the
health of the public, research on any serious hypotheses about
their safety are important to pursue. Several studies are underway
to investigate still unproven theories about vaccinations and
severe side effects.
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