Sep 22 2009

More Evidence for Vaccine Safety

The BBC reports today of a National Health Service study that shows that autism rates are consistent at about 1% among all age groups. If true, this has profound implications for the now-discredited notion that autism rates are rising and that this rise is linked to vaccines.

Starting in the late 1990’s fear swept the UK over the MMR vaccine and a possible link to autism – sparked by research by Andrew Wakefield. In the decade since, a succession of studies have shown that Wakefield’s original research was not valid and that there is no detectable link between vaccines and autism. Attention shifted, especially in the US, from the MMR vaccine to a preservative, thimerosal, in some childhood vaccines. This prompted the Centers for Disease control and the FDA, to mitigate public concerns, whether justified or not, to remove thimerosal from the vaccine schedule, which they did by 2002 (except for insignificant trace amounts). This dramatic decrease in thimerosal exposure had no effect on the rate of increase of autism diagnoses – pretty much the nail in the coffin of the thimerosal-autism hypothesis.

Unfortunately – this is a fake controversy, not a genuine scientific one, and dedicated anti-vaccinationists are not swayed by compelling evidence, so their vaccine-autism fear mongering continues, and has even been increasing. They have migrated their claims away from MMR and thimerosal to other “toxins” (mostly imagined), and made-up claims such as vaccines overwhelm children’s immune systems (when in fact they represent an modest addition to the antigens kids are exposed to).

The final objection of the anti-vaccine crowd is that there has never been a study comparing never-vaccinated children to vaccinated children in a prospective randomized trial. Of course, this study will never be done and should never be done because it is unethical to randomize children to not receive standard preventive treatment.

In medicine there are some studies that simply cannot be done. There has never been a study where subjects were randomized to either smoke or not smoke, so all data on the links between smoking and cancer are based on animal data or are epidemiological. The limitation of epidemiological data is that it cannot control for all possible variables, only all known variables. Randomizing large numbers of subjects is the only way to control for all possible variables.

This limitation has opened the door for the tobacco industry to claim for decades (although they have quietly dropped this claim) that smoking does not cause lung cancer – both lung cancer and smoking are caused by “factor X” – one of those hidden variables that epidemiology cannot control for.

While this reasoning is technically correct – we cannot rule out “factor X”- it misses an important feature of epidemiology: the ability to triangulate to the most probable causal relationship by doing multiple epidemiological analyses. With regard to smoking and lung cancer, for example, epidemiology has found that risk of lung cancer increases with duration of smoking, with inhaling deeply, with smoking unfiltered tobacco, and decreases after quitting smoking. There is also biological plausibility as tobacco smoke contains known carcinogens.

What about vaccines and autism? First, the premise that autism rates have been increasing is likely not true. While a real small increase cannot be ruled out by the data, the observed increase in diagnostic rates can be explained based upon increased surveillance and a broadening of the definition – in fact autism is now referred to as autism spectrum disorder. This hypothesis makes a number of predictions – for example if the definition of autism has increased then there must be diagnostic substitution, children that are now diagnosed with autism would have been diagnosed with something else (like non-specific language disorder) 20 years ago. This turns out to be true.

The hypothesis that autism rates are not truly increasing makes several predictions (like the hypothesis that smoking and lung cancer are linked because smoking causes lung cancer). In addition to diagnostic substitution, it predicts that if you apply the same diagnostic criteria to a cohort of children now and a cohort of children in the past, the incidence of autism should be the same. This also turns out to be true.

Now we also get to the current NHS study – another prediction is that if autism rates are not truly rising, then if you apply the same diagnostic criteria to people of different age groups the prevalence of autism should be the same. The caveat to this is that even if autism incidence is the same across time, some people may lose their autism diagnosis as they age or autism may be associated with a lower life expectancy.

However, despite these concerns, the NHS looked at 7,500 adults and found a prevalence of autism of about 1% in all age groups – the same as the current rate among children. This is consistent with the hypothesis that autism rates are in fact stable over time – not increasing.

This is vital blow to the vaccine-autism hypothesis, because the vaccine schedule has been increasing over the last 20 years, the MMR was introduced in the early 1990s, and thimerosal exposure has risen and then fallen to almost nothing. Throughout all of these changes, autism rates have remained stable. This is important because in order to demonstrate toxicity you need to demonstrate a dose-response – the higher the exposure to the alleged toxin the greater the risk or severity of the disease or disorder you think is caused by the toxin. This burden of proof was met for smoking and lung cancer – there is a clear dose-risk response. This is no detectable dose-risk link between vaccines and autism.

Of course, this one study is not definitive. While 7,500 subjects is enough to do statistical analysis, it is a modest-sized study, and replication with larger numbers is welcome.

The confidence of the scientific community that vaccines are generally safe and that they do not contribute to the risk of autism is built not on any single study but on a triangulation of many studies, all pointing to a lack of a connection. So far, no matter how we choose to ask the question or look at the data, there is no detectable connection between vaccines and autism. There will always be other kinds of data we don’t have that the anti-vaccinationists can point to, just as with the tobacco industry and smoking. But both are simply engaged in denial of the data we do have for ideological reasons.

So while we can throw this new study on the pile of those that are consistent with a lack of association between vaccines and autism, I don’t expect the anti-vaccine crowd to lose any sleep over their cognitive dissonance. They already have a well-insulated belief system that will be unaffected by this or any other data.

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