The MMR vaccine and autism
BMJ Group Medical Reference
Why the worry?
A 1998 study raised questions about the combined measles, mumps, and rubella vaccine (the MMR vaccine) and autism.  The researchers thought there might be a link between someone having the vaccine and getting a problem with their bowels. This bowel problem might cause a new kind of autism. But, in the end, the researchers said that their study didn't prove any such link.
The study has now been discredited and was withdrawn in 2010 by the medical journal that published it.  Even at the time, other researchers said the study was not done well.  It was small and only looked at 12 children. But the study showed up in newspapers and on TV. Some parents decided not to give their children the MMR vaccine. What does the evidence say?
It can be hard to prove beyond any doubt that a vaccine is safe. This is because millions of children get vaccines. And some of these children later get diseases. But this doesn't mean vaccines caused the diseases.
To prove a link between the MMR vaccine and autism, we would need to do a type of study called a randomised controlled trial. In a study like this, some children would get the vaccine and some would not.
But this study would be hard to do, for several reasons. We know that vaccines help a lot in stopping infections. So it would be unfair to not give some children the vaccines. Also, the study would have to go on for many years to spot any problems that take time to show up.
Other types of studies have given us reliable evidence about the MMR vaccine and autism. Two types are cohort studies and case control studies.
In cohort studies, researchers have looked at a large group of people, usually many thousands, to see how many got autism. Then they have checked how many of these people have had the MMR vaccine.
In case control studies, researchers have looked at two groups. One group has autism. The other group does not have autism. Otherwise, the two groups are very similar. The researchers have then checked which people have had the MMR vaccine.
There have been many of both of these types of studies on the MMR vaccine and autism. There is no evidence of a link between the MMR vaccine and autism. 
Here is what these studies show.
Although there has been a big rise in autism, there has only been a small increase in MMR vaccination over the same period:
The number of kindergarten children in California with autism rose by almost 400 percent from 1980 to 1994.  But this wasn't linked to a similar increase in MMR vaccination
Although autism increased among boys in the UK from 1988 to 1993, the number of children vaccinated with MMR stayed about the same.