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.

12 responses so far

12 thoughts on “More Evidence for Vaccine Safety”

  1. juga says:

    The situation in the UK was not helped by the reaction of the government to parents’ fears. Because we have a National Health Service funded from taxation, the government has a very direct say in what happens. Many parents felt that the combined MMR vaccine might be more risky than individual vaccines (measles, mumps and rubella). That may not have been true but the government would not permit (i.e. pay for) separate vaccinations for parents who wanted it. The reason might have been cost or it might have been fears about uptake but this was never explained to the satisfaction of many people.

    Secondly, because we do not have a very open government, there was a suspicion that, at a national level, an increased level of autism might be acceptable when judged against a decrease in the consequences of the diseases themselves. e.g. Could we trust our politicians not to judge that a few more cases of autism were preferable to a number of deaths from measles, for example? Again, the government did not really tackle concerns like this, however unreasonable they might have been.

    Thirdly, it is my understanding that vaccines work by conferring “herd immunity”. i.e. When more than a certain percentage of the population is vaccinated, the disease does not take hold and spread. The British government never, to my knowledge, put this forward as the reason children should be vaccinated. The consistent message was if you don’t have your child vaccinated, it might die. If they had appealed to the social responsibility to protect everyone, they might have had more success. Most people’s experience of these diseases in their own childhood was not that some children died, so the message was not readily accepted.

    It is an interesting ethical question how much you should protect your children from low risks of severe consequences, especially when the “protection” may not be risk free itself. I don’t think parents will ever agree on this so the only answer for vaccination is surely to emphasise openly the benefits to everyone, rather than trying to frighten people.

  2. juga – while herd immunity is important, the stats also show that for the individual the benefits of vaccines outweigh the risks, even for now rare diseases. This is because the risks are very low. Herd immunity is a bonus.

    So there is really no dilemma – the evidence supports the decision to get vaccinated, unless there are special medical concerns for an individual. And that is where herd immunity really comes in – to protect those who cannot get vaccinated.

  3. jonny_eh says:

    This study and blog post couldn’t have appeared at a better time. Just this morning in my local paper I read a story about the H1N1 vaccine resurrecting autism fears. They quote a worried parent who uses the exact argument debunked in this blog post (that no observational studies have been done on the two groups). Thanks to this blog post, I was able to send in a nicely informed letter to the editor referencing the NHS study.

    Thanks Dr. Novella!

  4. Enzo says:

    “H1N1 vaccine resurrecting autism fears”

    …Along with the rise in the belief that flu shots cause the flu or can result in debilitation or even death. These risk factors are miniscule compared to the benefits, but media hype and random rumor abound.

    New York just passed a law requiring flu vaccination for all health care workers with patient contact, and already I’ve heard employees and even nurses saying they will try to find a way to avoid vaccination due to these concerns. Strangely, these are the same people that are on expensive extract supplements for cholesterol. If this kind of thinking can persist in a health care environment, we are REALLY doing a poor job of getting the right information out there.

  5. Neuroskeptic says:

    juga – The British government could have handled the situation better, in hindsight, but have some sympathy – they were dealing with a fairly bizarre situation, and given that many British people will never believe a word the government says anyway, I’m not sure they could have succeeded in persuading people.

    But one huge mistake, in my view, was Tony Blair’s refusal to disclose whether his own son had had the vaccine. He claimed it was a matter of personal privacy but once the media started speculating he should have swallowed that pride and just said “Yes, he had it”. Because as it stands it looked to everyone like he hadn’t (and maybe he hadn’t, we don’t know) and that was a PR disaster for MMR.

  6. Now that I think about it, it really sounds like the situation in Britain was at least a partial inspiration for Torchwood: Children of earth.

    Any Torchwood fans out there that agree?

  7. SGUfanCPH says:

    I just read a news article from 2003 on the Danmarks Radio website:

    (late night, my edits of grammar from google translate might be wobbly. Link to original news story at the end).

    “No link between mercury and autism
    3. September 2003 10:09

    A Danish research team can be the first in the world finds no link between mercury in vaccines and autism in children.

    On the contrary, as the number of children with autism rose after mercury in vaccines was removed.

    Mercury was used as a preservative in vaccines.

    Surprising result
    The researchers had decided that they would investigate allegations that mercury compound thimerosal, which has been used to preserve vaccines, can increase the risk of children developing autism.

    But instead of finding a decrease in the incidence of autism since thimerosal was phased out of childhood vaccines, the result was the opposite. The number of children who were diagnosed with autism rose.

    Research Fellow Kreesten Madsen of the Department of Epidemiology and Social Medicine at Aarhus University has led the study, published in the American medical periodical Pediatrics.

    He believes that the rise in autism could be because we are getting better at diagnosis and that altogether there has have been more focus on the disorder.

    Mercury until 1990
    This study is based on information from Danish registries for vaccines and psychiatric diagnoses.

    956 children in Denmark were diagnosed with autism in the years 1971 to 2000.

    Until 1990, when thimerosal was used in Danish vaccines, there was no increase in the incidence of autism.

    But from 1991 more and more children were diagnosed with autism.

    Denmark was the first country in the 1992 phase out thimerosal as a preservative in vaccines.

    No mercury in vaccines umbrella
    Thimerosal was used in Denmark for only two childhood vaccines, di-te-pol and the Hib vaccine, which protects against meningitis.

    Thimerosal has never been used in the umbrella MMR vaccine, because it would destroy the vaccine.

    Kreesten Madsen and his colleagues could a year ago to publish an investigation of links between MMR vaccine and autism.

    It built on more than half a million children, and it could not demonstrate any increased risk by being vaccinated.”

    So clearly, the discussion should have ended in 2003. Right?

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