The study, by researchers at the Center for Disease Control and Prevention, found no connection between the number of vaccines a child received and his or her risk of autism spectrum disorder. It also found that even though kids are getting more vaccines these days, those vaccines contain many fewer of the substances that provoke an immune response.

The study offers a response to vaccine skeptics who have suggested that getting too many vaccines on one day or in the first two years of life may lead to autism, says Frank DeStefano, director of the Immunization Safety Office of the CDC.

To find out if that was happening, DeStefano led a team that compared the vaccine histories of about 250 children who had autism spectrum disorder with those of 750 typical kids. Specifically, the researchers looked at what scientists call antigens. An antigen is a substance in a vaccine that causes the body to produce antibodies, proteins that help fight off infections.

The team looked at medical records to see how many antigens each child received and whether that affected the risk of autism. The results, published in The Journal of Pediatrics, were unequivocal.