Vaccines and autistic spectrum disorders are an emotional subject. The two were originally linked in a study that has since been retracted. A new study from the Centers for Disease Control (CDC) will hopefully lay to rest lingering parental fears about vaccinating their children.

Even though the British medical journal, The Lancet, retracted the study linking autism (as it was then called) to vaccination in 2010, parents' fears have been harder to change.

Researchers compared the total number of vaccine antigens that children in each group received during the first two years of life and the maximum number of antigens the children received during a single office visit. The number of antigens was the same in each instance.

Retracting a scientific study is not something that is done lightly. The authors of the retracted study stated they had found no evidence that vaccines caused autism. But when one author mentioned to the press that there was still a faint correlation between vaccines and the incidence of autism, the media in Britain and elsewhere ran with that. Understandably, parents became alarmed by the message.

Ten of the 13 authors of the retracted study had disavowed it. Without any other explanations for the roots of autism, many parents have continued to believe — or at least fear — the possibility that vaccines are somehow behind autism. You can read the details of the retracted study (which was of only 12 children) here.

Autism spectrum disorders (ASDs), as autism is now known, tend to be noticed around the same time that babies are receiving a series of vaccinations during their first two years of life. It's natural that scientists and parents would want to see if there was a connection.

The CDC study looked at the health records of over 1000 children: 256 vaccinated children with an ASD and 752 vaccinated children without an ASD. Researchers compared the total number of vaccine antigens the children in each group received during the first two years of life and the maximum number of antigens the children received during a single office visit. The number of antigens was the same in each group.

The study found no link between the vaccinations children received before their second birthday and autism. "Our findings showed that the number of antigens, the molecules in vaccines that stimulate the immune system to protect against infectious diseases, received in one office visit or cumulatively over the first two years of life is unrelated to the development of ASDs," Frank DeStefano, lead author of the study, told The Doctor.

The study is published online in Pediatrics and is freely available for parents to read.

"I would definitely tell [parents] that vaccinating their child according to the recommended schedule is one of the most important things that they can do to maintain their child’s health," says DeStefano, who is also director of the immunization safety office at the CDC.

Parents concerned about overwhelming a child’s immune system with vaccines need to consider the context of all the antigens and immunological stimuli that children are exposed to normally throughout early childhood, DeStefano cautioned. "So things like the number of antigens and the immunological stimulation that you get from vaccines on top of that are not that much." Such stimuli include bacteria, viruses, and substances in the environment and are one of the reasons why finding what, if any, exposures or environmental factors cause autism is so difficult.

ASDs appear to have a strong genetic component. It is becoming apparent that structural changes in the brain that manifest in autism likely occur before the child is born.