FAQs - Prevalence, Autism, DD, NCBDDD, CDC

Prevalence of ASDs
It is clear that more children than ever before are being classified as having autism spectrum disorders (ASDs). But, it is unclear how much of this increase is due to changes in how we identify and classify ASDs in people, and how much is due to a true increase in prevalence. By current standards, ASDs are the second most common serious developmental disability after mental retardation/intellectual impairment, but they are still less common than other conditions that affect children’s development, such as speech and language impairments, learning disabilities, and attention deficit/hyperactivity disorder (ADHD). The impact of having a developmental disability is great for the families affected and for the community services that provide intervention and support for these families. It is important that we treat common developmental disabilities, and especially ASDs, as conditions of urgent public health concern, do all we can to identify children’s learning needs, and start intervention as early as possible to give all children the chance to reach their full potential.

What is the difference between “incidence” and “prevalence”?
Incidence is the number of new cases of disease in a defined group of people over a specific time. Prevalence is the number of existing disease cases in a defined group of people during a specific time period. Public health professionals use prevalence measures to track a condition over time and to plan responses at local, regional, and national levels. Incidence is very difficult to establish because the exact time a person develops an ASD is not known.

What is the prevalence of autism?
CDC’s Autism and Developmental Disabilities Monitoring (ADDM) Network released data in 2007 that found about 1 in 150 8-year-old children in multiple areas of the United States had an ASD.

Since the ADDM sites do not represent a nationally representative sample, the prevalence estimates should not be generalized to every community in the United States. Although accurate for the areas we studied, rates may be higher or lower in other areas. However, these prevalence estimates can help communities project how many children may have autism for planning and identification purposes. They can also be used to provide for more appropriate interventions for children with ASDs.

What do the ADDM network results tell us about the prevalence of ASD in the United States?
Results from CDC’s ADDM network showed the average ASD prevalence among states participating in the project was 6.7 per 1,000 children in 2000 (6 sites) and 6.6 per 1,000 in 2002 (14 sites), or approximately 1in 150 children. Most sites identified between 5.2—7.6 per 1,000 8-year-old children with ASD in 2000 and 2002. There was some variation with ASD prevalence significantly lower in 1 site (3.3 in AL) and higher in 1 site (10.6 in NJ). Prevalence was stable from 2000 to 2002 in 4 of the 6 sites that participated in both surveillance years, but increased slightly in GA and significantly in WV, indicating the need for ongoing monitoring of prevalence over time. These data provide important information on the prevalence of ASD in areas of the United States and will be used to examine trends in the occurrence of these disabilities over time.

What did we know about the prevalence of Autism Spectrum Disorders (ASDs) before ADDM?
For decades, autism was believed to occur in 4 to 5 per 10,000 children. In 2004, CDC partnered with the American Academy of Pediatrics (AAP) to issue an Autism A.L.A.R.M.^[1] At that time, Data from several studies that used the current criteria for diagnosing ASDs (DSM-IV and ICD-10) found prevalence rates for ASDs between 2 and 6 per 1,000 children. Therefore, it was summarized that up to 1 in 166 children (6/1,000) have an ASD. While the 1in166 indicated the average upper estimate, there have been studies that have found as many as 12 in 1,000 children with an ASD in Europe and Scandinavia. Studies conducted specifically in the United States have found rates between 2 to 7 per 1,000 children in the past decade.

How many children in the United States have an ASD?
There is not a full population count of all individuals with an ASD in the United States. However, using the prevalence data stated above, we can estimate that if 4 million children are born in the United States every year and assuming the prevalence rate has been constant over the past two decades, we can estimate that up to 560,000 individuals between the ages of 0 to 21 have an ASD. However, many of these individuals may not be classified as having an ASD until school-age or later. Since behaviors related to the ASDs are usually present before the age of 3 years, it is important to make sure the individuals are being identified and are receiving appropriate intervention services as early as possible.

How many children with ASDs are being served through public special education programs?
In 2006, 224,594 children ages 6-21 and 35,111 children ages 3-5 were served under the “autism” classification for special education services^[2]. Not all children with an ASD receive special education services under the classification of “autism,” so the education data underestimate the actual prevalence of ASDs. For more information about children served through public education programs, go to the IDEA data website.

Has the number of children being served under an ASD classification in public special education programs changed?
Yes. Between 1994 and 2006, the number of 6 to 17-year-old children classified as having an ASD in public special education programs increased from 22,664 to 211,610.^[2]

While it is clear that more children are getting special education services for autism than ever before, it is important to remember that this classification was only added in the early 1990s. Growth in the number of children classified may be caused in part by the addition of autism as a special education category.

Why do we need to know the prevalence of ASDs?
Families, public agencies, and some private agencies bear the responsibility of paying for costly services involved in educating and treating symptoms associated with ASDs. Accurate reporting of ASD prevalence can help people plan for the resources needed such as therapies, trained teachers, diagnosticians, health care providers, and related service professionals. A recent study (Ganz 2006) indicated that the economic costs associated with autism are approximately $35 billion dollars per year; not to mention the untold challenges that families face in understanding their family member with ASD. Planning for housing and job training also require accurate numbers over time. Accurate reporting of ASDs over time and from different communities can also help to answer the question, is their variability in ASDs in different areas of the country or in different groups of people? Precise reporting of ASDs leads to better planning by service providers, and helps to provide justification for limited research dollars from public and private sectors. Precise reporting of ASDs can also lead to greater awareness, encourage commitment by service providers and researchers, and help lead the way to more effective intervention and prevention.

What is CDC doing to understand the prevalence of ASDs in the United States?
CDC has set up the only collaborative network to determine the prevalence of the ASDs in the United States. The Autism and Developmental Disabilities Monitoring (ADDM) Network is made up of scientists working in 10 sites across the nation, in addition to CDC. Metropolitan Atlanta Developmental Disabilities Surveillance Program (MADDSP).

How do the rates of ASDs in special education compare with those of other special education categories?
In 2006, according to IDEA administrative counts, 6,580.679 children children ages 6–21 received services through 13 categories in public special education programs. Specific learning disability was the most frequent education category identified, and speech and language impairment was second. Together, these two categories made up nearly 65% of all special education placements. The mental retardation classification made up about 9% (523,240). Autism made up about 4% (224,594).^[2]

CDC’s Metropolitan Atlanta Developmental Disabilities Surveillance Program (MADDSP ) found the autism rate among 8-year-old children in 1996 to be 4.2 per 1,000.^[3] That’s lower than the rate for mental retardation (15.5 per 1,000) but higher than the rate for cerebral palsy (3.6 per 1,000), hearing loss (1.4 per 1,000), and vision impairment (1.4 per 1,000) found among children of the same age.^[4]

How do ASD rates compare with rates of other disabilities and diseases diagnosed in children?
Down syndrome, the most commonly identified cause of mental retardation, occurs in about 1 in 800 births.[5] Juvenile diabetes, a common childhood disorder, occurs in about 1 in 400 to 500 children and adolescents,[6] which is similar to the lower range of the ASD rates of 1 in 500. However, ASDs are more common than childhood cancer, which has a prevalence rate of 1.5 per 10,000 children, according to the National Cancer Institute.[7]

Links
Autism Fact Sheet (National Institute of Neurological Disorders and Stroke)

Disclaimer: We have provided a link to these sites because they have information that may be of interest to you. CDC does not necessarily endorse the views or information presented on these sites. Furthermore, CDC does not endorse any commercial products or information that may be presented or advertised on these sites.

References
[1] American Academy of Pediatrics, The National Center of Medical Home Initiatives for Children with Special Needs: http://www.medicalhomeinfo.org/health/Autism%20downloads/AutismAlarm.pdf

[2] Individuals with Disabilities Education Act (IDEA) Data. Number of children served under IDEA Part B by disability and age group, 2006. [cited Jan 2008]. Available at: https://www.ideadata.org/arc_toc8.asp#partbCC.

[3]Metropolitan Atlanta Developmental Disabilities Surveillance Program. [cited 2006 Nov]. Available at: http://www.cdc.gov/ncbddd/dd/ddsurv.htm#prev.

[4] Karapurkar Bhasin T, Brocksen S, Nonkin Avchen R, Van Naarden Brau, K. Prevalence of four developmental disabilities among children aged 8 years - The Metropolitan Atlanta Developmental Disabilities Surveillance Program, 1996 and 2000. MMWR Morbidity and Mortality Weekly Reports 1996;55 (SS01):1-9.

[5] National Center on Birth Defects and Developmental Disabilities. Key Findings from Recent Birth Defects and Pediatric Genetics Branch Projects. [cited 2006 Nov]. Available at: http://www.cdc.gov/ncbddd/bd/ds.htm.

[6] National Center for Chronic Disease Prevention and Health Promotion. National Diabetes Fact Sheet. [cited 2006 Nov]. Available at: http://www.cdc.gov/diabetes/pubs/estimates.htm#prev2.

[7] Gloeker Ries LA, Percy CL, Bunin GR. Cancer Incidence and Survival among Children and Adolescents: United States SEER Program 1975–1995. National Cancer Institute. [cited 2005 Jan 21]. Available at: http://seer.cancer.gov/publications/childhood.