There are several reasons why some parents choose not to vaccinate their children, though most of them come down to misinformation from anti-vaccine advocates. With the recent addition of Jenny McCarthy to the popular ABC television show The View, many people are wondering what effect, if any, this will have on vaccination rates.
Much of the current fear and doubt about the safety of vaccines and autism can be traced back to Dr. Andrew Wakefield, the lead author of a small 1998 case report suggesting a link between vaccines and the onset of childhood autism. The British General Medical Council found he had acted unethically in his research, and his paper, which was championed by several celebrities including McCarthy, was retracted by its publisher, the Lancet.
It is true that there are risks involved in vaccinations, as there are with any drug or medical intervention. The risks are not hidden but instead well-known and easily available from your doctor or online. The risks of side effects, however, are far less dangerous than the risks of catching the disease.
One way to encourage vaccination is to counter obvious examples of anti-vaccine misinformation. For example many people claim that it is not the deactivated measles-mumps-rubella (MMR) vaccine itself that causes autism but instead a preservative used in the mix called thimerosal, which contains potentially toxic organic mercury. However as science journalists (including Seth Mnookin, author of The Panic Virus: A True Story of Medicine, Science, and Fear) noted on the PLoS blogs, the MMR vaccine "does not and never did contain thimerosal." Furthermore, thimerosal hasn't been in any American vaccines for nearly fifteen years.