Autism spectrum disorder and childhood vaccinations: in a nutshell
We don't know exactly what causes autism spectrum disorder (ASD), but there are many theories.
Many of these theories are unproven, with little or no scientific evidence behind them. One of these unproven theories is that ASD is caused by childhood vaccinations, specifically the measles-mumps-rubella (MMR) immunisation. Another theory is that the mercury-based preservative (thiomersal) that was once used in vaccinations is to blame.
Although these issues were of particular concern in the 1990s and early 2000s, many studies have been conducted since then. We now know that there is no scientific evidence that either the MMR vaccine or mercury is involved in the development of ASD.
In other words, vaccinations are not associated with the development of ASD.
There is no one proven answer to the question of what causes ASD. It is clear, however, that what parents do or don't do does not cause ASD in their child.
MMR and autism spectrum disorder: discredited theories
There are several - often conflicting - theories about an association between autism spectrum disorder (ASD) and vaccines, including MMR.
One theory is based on the idea that ASD might be caused by too much opioid activity in the brain.
Opioids are natural chemicals in the body, which have an effect on pain similar to morphine. The excess opioid theory suggests that in the bowels of children with ASD, some proteins are not properly digested. This results in the release of chemicals (called exorphins) that can end up in the nervous system. It is claimed that, once there, these chemicals cause an increase in opioid activity.
Some people have suggested the MMR vaccine might cause a persistent measles infection in the gut, leading to 'leaky-gut' syndrome. This allows partially digested proteins to pass through the gut into the bloodstream, and from there to the brain. This can lead to increased opioid activity and thus to ASD.
Other theories include the idea that the measles virus itself invades the brain. It then triggers an immune response resulting in inflammation in the brain, causing ASD.
Wakefield: the controversy
In 1998, researcher Andrew Wakefield and his colleagues published a paper in the medical journal The Lancet describing an apparently new syndrome linking developmental disorders (like ASD) and bowel problems in children who had previously been developing typically.
In 8 of only 12 cases studied, parents linked the beginning of the behaviour difficulties with the MMR vaccination.
The researchers stated that they did not prove a link between the MMR vaccine and the new syndrome. But their paper discussed the proposed link extensively. The paper suggested that the combined MMR vaccine was implicated in the development of ASD, although the single measles vaccine was not.
After the paper was published, Dr Wakefield publicly discussed the link. He suggested there was a case for splitting the vaccine into its component parts.
Criticism of the research
Since 1998, Dr Wakefield's research has been criticised for several reasons, including the following:
- The research applied measures meant for adults to test results from children. This means that some of the findings about bowel disorders in these children were in fact normal for children.
- The paper published an unproved link between the new syndrome that Dr Wakefield described and the MMR vaccine.
Ten of the paper's authors issued a partial retraction in 2004. They suggested that the link between ASD and bowel disorders is worthy of further investigation. But they admitted they did not find that the MMR vaccine caused ASD.
In July 2007, the General Medical Council began an inquiry into claims of professional misconduct against Dr Wakefield and two colleagues. The claims included that he:
- subjected children to unnecessary tests
- was being paid at the time to advise solicitors on legal action by parents against the makers of the MMR vaccine
- had links with a single measles vaccine patent.
In January 2010, the General Medical Council ruled that the charges against Dr Wakefield were proven and that he had acted dishonestly and irresponsibly. Following the ruling, The Lancet retracted the Wakefield paper.
In May 2010, Wakefield was struck off the medical register.
Several large-scale studies have found no evidence that the MMR vaccine causes ASD.
Studies that disprove links between MMR and autism spectrum disorder
One study of more than 500 000 Danish children found no increased risk of ASD among those who had received the MMR vaccine compared with those who had not.
Another study of more than 27 000 Canadian children noted that rates of pervasive developmental disorder (PDD) increased over time. But this happened when the rate of MMR vaccination was going down, which means the MMR vaccine was not causing the cases of ASD.
Other researchers found that rates of ASD continued to rise in a region of Japan - even after the MMR vaccine was stopped. Again, this suggested that the MMR vaccine was unlikely to be the main cause of ASD.
In an attempt to replicate part of Dr Wakefield's findings, researchers compared bowel tissue of 25 children with ASD and bowel disorders with 13 children with only bowel disorders. The researchers found no differences in the presence of the measles virus between the two groups.
Mercury (thiomersal) and autism spectrum disorder: another discredited theory
Blood contains several different chemicals in small amounts. But certain chemicals - like mercury - can be poisonous if the levels are too high. Some people claim that autism spectrum disorder (ASD) is caused by excess mercury in the blood, which the child's body can't get rid of naturally.
People make this claim because they believe that the characteristics of ASD are similar to symptoms of mercury poisoning. But there are also several characteristics of mercury poisoning that do not resemble ASD, like irrational fears, irritability and some movement disorders.
Supporters of this theory also suggest that the excess mercury comes from vaccines. This is because in the past, thiomersal (a chemical related to mercury) was used as a preservative to make some vaccines.
Others have suggested that thiomersal weakens the immune system, allowing the live virus contained in the vaccine to persist and damage the gut and brain.
Thiomersal-based vaccines are no longer used for children under eight in Australia. The only exception is one hepatitis B vaccine. It contains such a small amount that doctors consider it to be insignificant.
A large-scale study found that children who had not been exposed to thiomersal had more cases of PDD. Another study found there was no reduction in the rates of ASD after thiomersal was removed from vaccines in California.