Vaccine
Safety > Issues of Interest > Autism
FAQs
(frequently
asked questions) about
MMR Vaccine & Autism
(Measles,
Mumps, and Rubella)
|
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. |
|
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)
Top
- 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.
Top
- 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.
Top
- 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.
Top
- 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.
Top
- 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).
Top
- 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.
Top
- 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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