Tuesday, August 26, 2014

Did the CDC hide a link between vaccines and autism?

So there's an iReport article going around the Facebook circuit accusing the CDC of covering up a link between vaccines and autism. The allegations from the author "eplettner" are pretty serious: this person accuses the CDC of covering up evidence that the MMR vaccine causes autism.

Are eplettner's claims of a coverup true, and more importantly, is there a link between the MMR vaccine and autism?

As many of my Facebook friends have disovered, I love correcting factually wrong statements, especially strong ones. I like to do this regardless of whether I agree with the conclusion of one side or the other.  On a controversial topic like whether or not vaccines raise the risk of autism, there are plenty of statements flying around as fact. Many of these statements directly contradict each other... in this case, "The evidence now is convincing that the measles–mumps–rubella vaccine does not cause autism or any particular subtypes of autistic spectrum disorder." (link from a site claiming to draw this conclusion from the CDC study) and "CDC recognizes an increased risk of autism from MMR." from the iReport.

Finding correct conclusions requires starting with true facts. Unfortunately, the internet and human interaction in general allows true and false facts to be spread equally quickly. To make things worse, humans have a tendency to accept claims that support what we already believe, while rejecting stronger claims that don't fit with our beliefs. This is known as confirmation bias.

So what's up with the CDC study?

The original CDC study is DeStefano et al. (2004). The study compared over 600 children with autism to non-autistic children to determine whether the time of receiving MMR vaccination was linked to autism. The study determined:
"Similar proportions of case and control children were vaccinated by the recommended age or shortly after (ie, before 18 months) and before the age by which atypical development is usually recognized in children with autism (ie, 24 months). Vaccination before 36 months was more common among case children than control children, especially among children 3 to 5 years of age, likely reflecting immunization requirements for enrollment in early intervention programs."

One person, Dr. Brian Hooker, believed vaccines gave his son autism. He got the original data from the CDC study, and reanalyzed it (Hooker, 2014) from the original study using a different statistical method and came up with similar data: no increased risk of autism from vaccines given before 18 months or 24 months, but slightly higher rates of autism for the group of individuals receiving vaccines before 36 months. Hooker, however, asserted the 36-month discrepancy was not due to parents getting their autistic kids ready for school.

Hooker also broke down the data by race and gender. While he only had a few African-American males in his study (which tends to make analysis unreliable), the interesting results were confined to this group, showing a 3.4x rate of autism in African-American boys vaccinated between 24 and 36 months. White males, and all females, showed no link between autism and age of vaccination.

I'm not a statistician, so I can't compare the two methods, but vaccine author David Gorsky has compared them. Skip to the section "Brian Hooker's "Reanalysis"." He notes that the CDC data set wasn't designed for the type of analysis Hooker did, and in particular, the number of African-American children was probably not enough to draw reliable conclusions about their group.

 So to answer the original questions:

Did the CDC cover up their data?

It seems unlikely that there was a "coverup" of a link between age of vaccination and autism, because DeStefano indicated this in the original study: a link between vaccines between 24 and 36 months and autism. So no, there was no coverup.

Is there a link between the MMR vaccine and autism?

DeStefano, pointed out that autism tends to manifest before 24 months, so parents of such kids often get their vaccines in right away to take advantage of school programs for their kids. This may explain the discrepancy, but this isn't certain, and we must rely on other studies to determine this.

Fortunately, many other studies have been done on MMR and autism. The best one is a Danish study that analyzed data on nearly the entire population of Danish babies over a period of 8 years and found similar rates of autism in vaccinated and unvaccinated children.  If age of vaccination did cause autism, then we would see fewer autistic children in the unvaccinated group, but unvaccinated children were diagnosed at similar rates. Even Hooker's reanalysis, flawed or not, reconfirmed that girls and white boys have no increased risk of autism due to age of MMR vaccine. There is no causal link between MMR vaccine and autism in the general population.

What about the claims in the iReport article?

The iReport article is titled "Fraud at the CDC uncovered, 340% increased risk of autism hidden from public". The article cites quotes from a Dr Thompson at the CDC, but it seems these quotes have come through Drs. Hooker and Wakefield via interview soundbytes. So are Thompson's quotes true - did CDC hide data? Dr. Hooker had to actually massage his data to even have enough African-American males to study as a subgroup. Hooker modified the pool of these boys from pre- 36 months to pre-31 months so that he'd have over five kids to look at. As Dr. Gorsky points out, "You cannot, and must not use small numbers to make big assertions." So to claim the CDC is hiding anything about African-American males is absurd - they followed good procedure and didn't report on something about which they had insufficient data. Yet the article claims that the original DeStefano study is "fraudulent" and cites a Change.Org petition and hashtag campaign to bolster the idea that their claims are true.

The article throws in other claims in an apparent attempt to prove a connection between vaccines and autism. In one such claim, it notes that autism is an admitted "secondary cause" of vaccine injury. The article fails to explain that this is a side effect of very rare injuries that don't statistically affect the rate of autism.

Conclusion - past and future research.

A good scientist seeks the truth, in this case, through careful study. Ever since the original claims of Andrew Wakefield that MMR vaccine causes autism - based on data that Wakefield falsified in an actual fraud - there have been many studies done on the possibility of a link. Both the original CDC study by DeStefano and the reanalysis by Hooker contradict Wakefield's claims of a general link, and other studies have researched other vaccines and possible vaccine-autism links and found nothing. The Hooker study provides a glimmer of possibility of a link between vaccination timing and autism in African-American boys, one that will doubtless be studied more thoroughly in the next ten years. When such a study is concluded, our human tendency will to either accept it if it agrees with what we believe, or deny it if it disagrees with what we believe. Fight this tendency. Don't let confirmation bias get in the way of finding the truth.

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