No link between autism and childhood vaccinations
We don’t know exactly what causes autism, but there are many theories. Some theories have good evidence, but other theories are unproven, with little or no scientific evidence behind them.
One of these unproven theories is that autism 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 autism.
In other words, vaccinations are not associated with the development of autism.
There’s no one proven answer to the question of what causes autism. But causes might include brain development and genetic factors.
MMR and autism: discredited theories
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 autism) 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 autism, 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 unproven 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 autism and bowel disorders is worthy of further investigation. But they admitted they did not find that the MMR vaccine caused autism.
In July 2007, the General Medical Council began an inquiry into claims of professional misconduct against Dr Wakefield and 2 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 autism.
Studies that disprove links between MMR and autism
One study of more than 500 000 Danish children found no increased risk of autism 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 autism.
Other researchers found that rates of autism continued to rise in a region of Japan – even after the MMR vaccine was stopped. Again, this suggested that the MMR vaccine was not the cause of autism.
In an attempt to replicate part of Dr Wakefield’s findings, researchers compared bowel tissue of 25 autistic children who had 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 is caused by excess mercury in the blood, which the child’s body can’t get rid of naturally.
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. But in any case, the mercury in this preservative was not the type that accumulates in the body and causes difficulties.
Thiomersal-based vaccines are no longer used. None of the National Immunisation Program (NIP) vaccines used in Australia contain thiomersal. In Australia, thiomersal has been removed from all routine childhood vaccines since 2000.
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 autism after thiomersal was removed from vaccines in California.