By Jeremy Laurance
A study tracking the incidence of autism in a city where the MMR vaccine was withdrawn has provided powerful evidence that the injection is not linked to the mental disorder.
Research on 30 000 children in Yokohama, Japan, showed that autism continued to rise after the triple MMR vaccine was replaced with single jabs.
Japan stopped using the MMR injection in April 1993 because the mumps element of the vaccine was causing cases of meningitis. The MMR vaccine used in the UK contained a different mumps element.
A study by Hideo Honda, of the Yokohama Rehabilitation Centre, and Professor Michael Rutter, of London's Institute of Psychiatry, found the number of children diagnosed with autism by the age of seven doubled after MMR was withdrawn.
In the The Journal of Child Psychology and Psychiatry, the researchers say MMR "cannot have caused autism in the many children with autism spectrum disorders in Japan who were born and grew up in the era when MMR was not available."
Fears about MMR surfaced in the UK in 1998 after Dr Andrew Wakefield, from the Royal Free Hospital in London, claimed that the vaccine might trigger autism. The findings were based on a study of just 12 children and were later retracted by most of Dr Wakefield's co-authors.
Not one epidemiological study has revealed a link between the vaccine and autism. But until now, every one has focused on what happened after the triple jab was introduced.
In contrast the Japanese researchers looked at autism rates after the vaccine was replaced with single jabs. Dr Honda's team checked the records of 31 426 children born in one district of Yokohama between 1988 and 1996. The researchers counted how many children were diagnosed as autistic by the age of seven. They found that the number of cases continued to multiply after the withdrawal of MMR.
Rates ranged from 48 to 86 cases per 10 000 children before withdrawal, to between 97 and 161 per 10 000 after. The pattern was the same for the particular form of autism linked to MMR by Dr Wakefield, which causes children to regress suddenly after a period of normal development.
The cause of rising autism rates around the world remains a mystery. Some experts believe so-far unexplained environmental factors are responsible. Others disagree, saying the apparent rise could simply be the result of changing diagnostic criteria and increased awareness of the disorder.