A child's risk of autism is not increased by receiving too many vaccines too soon, a new study indicates.

According to US researchers, some parents continue to be wary of certain vaccines and/or the timing of these despite the fact that scientific evidence suggests that vaccines do not cause autism.

Much of this fear can still be traced back to a 1998 study by UK scientist, Dr Andrew Wakefield, which suggested that the MMR vaccine was linked to autism. The small study was published in the highly respected medical journal, The Lancet.

Since then, many parents have chosen not to have their children vaccinated with the triple vaccine. This is despite the fact that a number of studies since 1998 have found no such link with autism. Furthermore in early 2004, the editor of The Lancet, Dr Richard Horton, said that Dr Wakefield's study should never have been published as it was ‘flawed'.

Aside from the MMR vaccine, some parents are also concerned that providing a number of vaccines at the same time may ‘overload' their child's immune system, leading to autism. For example, babies receive a six-in-one vaccine at two, four and six months of age. These cover diseases such as diphtheria, polio and hepatitis B.

Some parents worry about giving a number of different vaccines on the same day, while others worry that children receive too many vaccines in the first few years of life.

The US researchers decided to investigate this further. They compared data from 256 children who had been diagnosed with autism spectrum disorder (ASD) and 752 children who did not have ASD. All of the children were born between 1994 and 1999.

Their analysis included details about the antigens the children had been exposed to via the vaccines. An antigen is a chemical that stimulates the immune system to produce resistance to infectious agents such as viruses and bacteria. In other words, they are the part of the vaccine that cause the immune system to produce antibodies in order to fight disease.

The researchers added up the number of different antigens in all vaccines each child received on a given day, as well as all the antigens they received via vaccines in their first two years of life.

They found that the total number of antigens found in the bodies of the children was the same whether they had ASD or not. In fact, no link between the number of antigens and autism could be found.

"The possibility that immunological stimulation from vaccines during the first one or two years of life could be related to the development of ASD is not well-supported by what is known about the neurobiology of ASDs," the researchers from the US Centers for Disease Control and Prevention concluded.

They also pointed out that that from the time of their birth, infants are exposed to countless antigens - via the air they breathe and the food they consume - as well as hundreds of viruses that are outside of vaccination.

Details of these findings are published in The Journal of Pediatrics.

For more information on childhood vaccination, see our unique Childhood Vaccination Tracker here