Autism-Vaccine Link: Evidence Doesn't Dispel Doubts
Many major medical groups say vaccines don't cause autism. Many parents say they do. So who's right?
Vaccine-Autism Disconnect: MMR Vaccine, Thimerosal
The main reason why anyone talks about vaccines and autism is that some parents have noticed changes in children shortly after the children were vaccinated. Their kids seemed to be developing normally, then suddenly stopped interacting with people and lost language abilities -- a condition called "regressive" autism.
Most medical researchers argue that this is probably a coincidence: Autism symptoms tend to become apparent around the same time that children are scheduled to get routine vaccines.
Although there are two separate issues concerning vaccines and autism, they're often lumped together. One has to do with the measles, mumps, and rubella (MMR) vaccine; the other involves vaccines containing the chemical preservative thimerosal, which contains a form of mercury that has been suspected of causing autism and has recently been removed from most vaccines.
The MMR scare started 10 years ago with a report published in The Lancet that described the cases of eight children who, as their parents recalled, developed autistic symptoms and digestive ailments shortly after getting their first MMR dose. The researchers proposed that the vaccine might trigger a previously unknown form of regressive autism. They suggested that maybe the measles virus in the vaccine lodged in the intestine, causing some kind of reaction that then affected the brain.
After that, experts studied whether the MMR vaccine could cause autism. To do that, they looked for clues among kids who did and didn't get the vaccine.
Since that initial finding, 14 studies including millions of children in several countries consistently show no significant difference in autism rates between children who got the MMR vaccine those who didn't.
The bottom line: It's very unlikely that the MMR causes autism, researchers say.
Vaccine-Autism Disconnect: Complex Science
A lot of the confusion comes from the complexity of the science involved in the issue. "It's hard to communicate in a way that's unambiguously clear," says Melinda Wharton, MD, deputy director of the CDC's National Center for Immunization and Respiratory Diseases.
Scientists often hedge about saying whether their findings prove or disprove anything. That's because the scientific method proceeds by constantly modifying theories rather than accumulating "proofs."