There’s no evidence that vaccinations have anything to do with the increasing incidence of attention deficit disorder (ADD) or its cousin,attention deficit hyperactivity disorder (ADHD). Both conditions have been diagnosed more frequently over the past two decades. (ADD is a learning disability that inhibits a child’s ability to concentrate and focus, while ADHD adds the characteristic of hyperactivity, making it hard for the child to sit still.
There has been discussion among experts about whether thimerosal, the mercury-based preservative used in some vaccines before 2001, contributed to the rise of ADD and ADHD. But experts have studied this possibility and found no evidence of a link. Three studies — two performed in the United Kingdom and one performed in the United States — found that thimerosal in vaccines did not cause developmental or psychological problems. And discontinuing the use of thimerosal in vaccines has not reduced the incidence of ADD or ADHD. So at this point, there’s no reason to believe that vaccines cause ADD or ADHD.