David J Carey
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Issues About Incidence and Prevalence of Autism
  
There has been so much written recently about the world-wide increase in the number of children with autism that this issue demands a bit of investigation. We will begin by looking at some of the publicity about the rising cases of autism.
 
In the 5/09/07 issue of The Californian found on the Internet the following statistics are quoted about enrolment of children with autism in elementary (primary) schools:
 
“In the Temecula and Lake Elsinore school districts, their populations have jumped by about 300 percent. In the Murrieta school district, the autistic enrolment has grown by 650 percent, an increase of about 100 students.”
 
In 2001 an article appeared on the Internet titled “The Geek Syndrome”. It looked at the incredible rise in cases of autism in silicon valley, a part of California known for its high achieve and high absorbed software developers. In the article the following is stated:
 
“In the past decade, there has been a significant surge in the number of kids diagnosed with autism throughout California. In August 1993, there were 4,911 cases of so-called level-one autism logged in the state's Department of Developmental Services client-management system. This figure doesn't include kids with Asperger's syndrome, like Nick, but only those who have received a diagnosis of classic autism. In the mid-'90s, this caseload started spiralling up. In 1999, the number of clients was more than double what it had been six years earlier. Then the curve started spiking. By July 2001, there were 15,441 clients in the DDS database. Now there are more than seven new cases of level-one autism - 85 percent of them children - entering the system every day.”
 
 
Figures release by the US Dept. of Education in noting the numbers of cases of autism diagnosed between 1992 and 2000 revealed the following statistics (IDEA is the federal law protecting children with disabilities to a free and appropriate education.

From the Department of Education annual reports to the US Congress

(Children with a diagnosis of autism aged 6-21] served by IDEA

(Individuals With Disabilities Education Act)

(Source: US Individuals With Disabilities Education Act data: http://www.IDEAdata.org/tables/ar_aa2.htm )
 
“In round terms, for every two children registered through IDEA with autism in 1992-93, there were almost eleven by 1999-2000.”

On Tuesday, 27/02/01 the BBC News issued a report that stated the cases of autism in the UK had increased ten-fold over the past decade.
 
What sense can we make of these statistics? Before attempting to look at the possible factors involved in this incredible rise in cases it is important to unpack some vocabulary. The terms “incidence” and “prevalence” connote the same concepts to the lay reader, yet they are not the same thing at all.
 
Incidence vs. Prevalence
 
Prevalence is a frequently used measure of how commonly a disease or conditions occurs in a given population. It measures just how much of the condition is present in a population at any given point in time. It is sometimes called “point prevalence”. Prevalence is calculated by dividing the number or persons with the condition at a particular point in time by the number of persons examined for the condition. Prevalence can be expressed as either a percentage or, if the number is low, as the number of cases per a given number of population (e.g. 10 in 10,000).
 
 Incidence measures the number of new cases of a condition. Incidence is usually calculated by the number of new cases of a condition (or disease) in a specified time period (usually one year) divided by the size of the population being studied who are initially free of the condition (or disease).
 
Any intelligent interpretation of the rise in autism must take into account the definition of incidence and prevalence outlined above.
 
What Accounts for the Rise in Autism?
 
Does the measles-mumps-rubella vaccine cause autism? Probably no question is more vexing to so many parents than this one. Fortunately, around the world there have been numerous studies looking for the answer. At present the weight of scientific evidence does not support the theory that the MMR vaccine causes autism. There is enough continued interest in the theory however to result in the Center for Disease Control in Atlanta, Georgia to continue to investigate.
 
What is the basis of this theory?
 
It was Wakefield, writing in the UK who first introduced statistics that linked the MMR vaccine to autism. Writing in the UK medical journal The Lancet, senior author Andrew Wakefield wrote a five-page “early report” suggesting that the MMR vaccine was associated with inflammatory bowled disease and regressive autism. (Regressive autism is a term for the type of autism associated with early normal development followed by the onset of autistic symptoms.)  The number of children studied by Wakefield and his associates was 12. Media attention was swift and world-wide. Headlines associated the vaccine with autism.

Since 16 July, 2007 Wakefield and his colleagues are facing charges of serious professional misconduct in relation to this study. For an interesting, in-depth read on this topic please see the following web page:
 
http://briandeer.com/mmr/lancet-summary.htm
 
Because the publicity surrounding Wakefield’s study was so massive (and a lot of it the direct result of the efforts of the hospital in which it was undertaken) parents world-side have been traumatised by wondering if they did not inadvertently cause their child’s autism. Also associated with the publicity surrounding the study was a marked decrease in the number of children being vaccinated. Vaccination rates in Britain are now running at about 80%. For “herd-immunity” to be present the vaccination rate needs to be about 92%.
 
Let’s look at the Facts
 
A study of 30,000 children in Yokohama, Japan in provides an interesting insight into the issue of MMR and autism. Prior to this study no investigation of the connection had ever been completed on a population who had no access to the vaccine. In Japan in the vaccine was removed in April 1993 because there was a fear it might be associated with meningitis (related solely to the anti-mumps component of the vaccine). This study looked at 31,426 children born in one district of Yokohama between the years 1988 and 1996. Only children diagnosed with autism by the age of 7 were included in the study. It was found that cases of autism continued to multiply after the vaccine withdrawal. Before the vaccine was withdrawn the number of cases was 48 to 86 per 10,000 children. After the vaccine was withdrawn the number of cases rose to 97 to 161 per 10,000! Interestingly, the same pattern was found for a group of children with a particular form of autism (normal development and then loss of function) that had been described by Wakefield in his original UK study. The study cannot rule-out the possibility that the vaccine causes autism in a small number of children but clearly indicates there is no large-scale cause and effect, at least in Yokohama, Japan.
 
In his book “Unstrange Minds: Remapping the World of Autism”, author Roy Richard Grinker undertook a world-wide investigating to explore the issue of rising cases of autism. His conclusion: there is no rise! The identification of autism depends on culture as much as on science (this is a position I myself have taken in earlier parts of this book.) He also reasons that more effective diagnosis, more attention to the condition in the media and better educational structures all account for the rise in autism cases worldwide.

 

Why do people continue to believe the autism-MMR link?
 
Autism can be accurately diagnosed by the age of three (some speculate that it can be diagnosed at age 1). This is about the same age as vaccinations are given to children. AS a result there will always be an apparent association between the vaccine and autism. The can easily be the result of chance alone. There is also considerable mistrust in the scientific community amongst lay people. These two factors combine with heart-wrenching personal testimonials from some parents along the lines of “Why won’t someone stop this madness?” that continue to decry the use of the MMR vaccine. It is only natural for people to search for answer but the apparent link between autism and the vaccine just has not born out under the weight of considerable scientific scrutiny.
 
What can we conclude?
 
Maybe it’s a bit early to come to any conclusion but some things are evident. Statistics can be misleading. Charts that purport to show a rise in autism must take into account some important factors: rise in population in the place being studied, change of diagnostic criteria over the last 15 years and the earlier age of diagnosis now commonly achieved.
 
Also the meaning of “incidence” and “prevalence” must be accounted. Certainly the incidence of autism is on the increase but that does not mean the prevalence is increasing. If you remain confused please re-read the section that defines the terms at the beginning of this chapter.

http://www.nctimes.com/articles/2007/03/11/news/californian/21_36_103_10_07.txt
http://www.wired.com/wired/archive/9.12/aspergers_pr.html

BBC News, 27 February 2001
The Lancet, volume 351, Number 9103, 28 February 1998
New Scientist, 17 February 2001
Grinker, R. R. (2007). Unstrange Minds: Remapping the World of Autism. New York: Basic Books