Oct 07 2009

Autism Prevalence

Two recent studies concerning the prevalence of autism in the US have been getting a lot of attention, because they indicate that autism prevalence may be higher than previously estimated. This, of course, fuels the debate over whether or not there are environmental triggers of autism.

One study was conducted by the CDC but has yet to be published. The results were announced ahead of publication by the US Health and Human Services Secretary Kathleen Sebelius to the autism community. She reports that the new prevalence of autism spectrum disorder (ASD) is now estimated at 1% or 100 in 10,000 children. This is an increase over the last few years. In 2002 the prevalence was estimated to be 66 per 10,000.

The second study was published in the journal Pediatrics and is a phone survey of 78,037 parents. They asked if they had any children who had ever been diagnosed with an ASD. Here are the results:

The weighted current ASD point-prevalence was 110 per 10,000. We estimate that 673,000 US children have ASD. Odds of having ASD were 4 times as large for boys than girls. Non-Hispanic (NH) black and multiracial children had lower odds of ASD than NH white children. Nearly 40% of those ever diagnosed with ASD did not currently have the condition; NH black children were more likely than NH white children to not have current ASD. Children in both ASD groups were less likely than children without ASD to receive care within a medical home.

These are slightly higher numbers than the CDC data. This is to be expected, however, since the Pediatrics study is a phone survey. Diagnoses were not confirmed by a clinician.

There is no question that the number of ASD diagnoses has been steadily increasing for the last two decades. The burning question has been – is this a real increase in the disorder or an artifact of increased surveillance and an expanded diagnosis. I have reviewed the evidence in depth previously – the evidence strongly supports the conclusion that the increasing autism prevalence is due to increased efforts to make the diagnosis and a broadening of the definition of autism. The evidence is not sufficient to conclude that there is not also a real increase in ASD incidence, but nor is there data to support this conclusion.

Further, since I wrote my last summary, there has been more data to support the conclusion that autism rates are really flat. The National Health Service also has recently published a survey of autism prevalence. While the US studies looked at children from 3 to 17, the NHS study looked at all age groups. Their question was this – is the prevalence of ASD the same or different among various age groups? If the incidence of ASD is truly increasing, then younger age groups should have more ASD than older age groups.

They found a consistent prevalence of 1% in all age groups they surveyed. This is remarkable for two reasons – first, they found the exact same 1% figure as the CDC US survey (assuming the CDC data is more accurate than the phone survey published in Pediatrics). This supports the conclusion that the 1% figure may be close to the true prevalence of ASD in the population.

Second, the NHS study found that the prevalence of autism was the same in all age groups, strongly suggesting that true ASD incidence has not been increasing over recent decades and supporting the increased surveillance and definition hypothesis.

Of course, the anti-vaccine community is exploiting these two new studies (the US studies – they have already found excuses and conspiracies to dismiss the implications of the NHS study). David Kirby, writing for Age of Autism, a notorious anti-vaccination site, wonders aloud why there is no alarm about these new ASD prevalence numbers. He is still playing his coy “conspiracy of silence” card.

It is also interesting that Kirby and others in the anti-vaccine community were predicting that autism rates should decrease following the near complete removal of thimerosal form the childhood vaccine schedule by 2002 in the US. Autism rates continued to rise at a steady rate, essentially killing the hypothesis that thimerosal is a significant contributor to autism. Now Kirby is citing the same evidence (continued rising autism rates) that punctured his previous hypothesis as support for his new hypothesis – the HepB vaccine.

That’s right – the anti-vaccine community has moved from the MMR vaccine to thimerosal to other “toxins” in vaccines and are now focusing their sites on the HepB vaccine. As each claim fails to be supported by scientific evidence, they simply migrate over to another claim – all with the core feature that vaccines are always to blame.

Meanwhile, the strong consensus of evidence is that ASD rates are not truly increasing at all.

Note: This entry was cross-posted over at Science-Based Medicine.

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