Prevalence and incidence make reference to a frequency measurement that divides affected and unaffected individuals within a population.  These measures work well when you can clearly distinguish between the affected and unaffected populations.  The presence of subclinical cases or blurred diagnostic lines, as in autism, requires a cautious interpretation of population statistics.

Prevalence refers to the total number of people having a condition at a given time and in a given population.  Incidence, by comparison, makes reference to the rate of newly diagnosed individuals.  Thus, incidence provides information about the risk of contracting a condition whereas prevalence indicates how widespread that condition may be. Prevalence includes all cases, both new and pre-existing.  People interested in examining cause and prevention usually target the incidence of a condition while those interested in looking at resource allocation and treatment planning prefer to examine prevalence.  Indeed, probably the main reason for using prevalence rates in autism is to allow federal agencies to distribute necessary resources and to justify funding allocations.  However, prevalence rates even for a chronic condition like autism, tend to be variable or intermittent. Prevalence taken at a single point in time (point prevalence) tends to underestimate the total frequency of a condition.  Longitudinal studies, like the one conducted by the CDC, with repeated and continuous assessments, are necessary for proper interpretation.

Reports from the CDC have shown an autism prevalence that has risen over the years (see figure below).  The suggestion is that the total number of diagnosed autistic people is increasing. The latest CDC study (2018) found a diagnosis of ASD of 1 in every 59 children (1.7%) with a gender ratio of 4:1 boys to girls.  The reported figure was a 15% increase over the CDC’s preceding study (published in 2016) showing a prevalence of 1 in 68 (1.5%).    As means of comparison, rare disorders are usually defined as affecting less than 1 in every 2,000 individuals while other common medical disorders, like coronary heart disease or diabetes, have prevalence rates of 6% and 9% respectively.  Autism falls between the extremes of rare and common disorders.

Are the reported increases in prevalence rates real?  The answer is a resounding yes, but the underlying cause(s) remains debated.  Indeed, another study funded and directed by the Health Resources and Services Administration (HRSA) Maternal and Child Health Bureau (MCHB), found an even higher prevalence rate of 1 in 40 children (2.5%).  Differences in study designs for how the CDC and HRSA studies were conducted help explain the variability in the reported prevalence rates.

The CDC study was part of the ADDMnet (Autism and Developmental Disabilities Monitoring Network).  The ADDM network is the CDC surveillance network active since 2000 in studying prevalence in over 300,000 8-year-old children in multiple communities.  It is not a nationwide study.  In 2014, there were 11 sites reporting: Arizona, Arkansas, Colorado, Georgia, Maryland, Minnesota, Missouri, New Jersey, North Carolina, Tennessee, and Wisconsin.  The age of 8 years was selected because by then most children with autism would have received a diagnosis (note: this is arguable as the spectrum of autism has broaden). Also, by 8 years of age the children would already be at school, meaning that schools would have helped in catching those children with a diagnosis and probably had educational records of them available. The latest CDC study provides for data collected in 2014.  Researchers at the different sites reviewed health and educational data and confirmed cases via record review.  There was no direct patient examination.  The data can’t be explained by a shift of diagnosis. Data collected used DSM IV criteria.  The prevalence rates varied by site (e.g., 1 in 70 in Arkansas to 1 in 35 in New Jersey) and depended on whether the individual sites had full access to educational records; being higher in those that had educational records. As an aside,  NJ has always been included and the prevalence in this state has always been high, exceeding and outpacing other states.  The reason for this is unknown as of present but some people hypothesize that it is the availability of quality healthcare provided by the state.

The 1 in 40 prevalence rate comes from data collected by the National Survey of Children’s Health (NSCH) for 2016 of children ages 3-17 years.  It was a national sample based on paper/web completed survey by parents.  Only 1 child per household participated. The data is publicly available http://www.childhealthdata.org/.  There was no case confirmation other than what the parents said.  Data, for other neurodevelopmental conditions, coming from this organization have consistently shown higher prevalence rates than other surveys.

The main findings of the report:

• The prevalence of parent-reported ASD among children aged 6-17 years was 2 percent in 2011-2012 compared to 1.2 percent in 2007.
• The change in prevalence estimates was greatest for boys and for adolescents aged 14 to 17 years.
• Children who were first diagnosed in or after 2008 were more likely to have milder ASD than those diagnosed in or before 2007.
• Much of the increase in the prevalence estimates from 2007 to 2011-2012 for school-aged children was the result of diagnoses of children with previously unrecognized ASD.

The prevalence rates for the CDC and NSCH were collected from different populations, age of participants, and types of screening.  The higher prevalence rate by the NSCH may be due to the use of parental reports, that unlike the CDC,  did not confirm a diagnosis.  Still, it is very clear that the prevalence rates are high and apparently increasing.