Despite concerns from some parents that the number of vaccines and the timing with which they are given to young children may contribute to autism risk, a new study backed by the U.S. Centers for Disease Control and Prevention finds otherwise.

Researchers looked at the number of antigens — the part of each vaccine that tells the immune system to create antibodies to fight disease — that children with and without autism received on any single day and over the course of their first two years of life. They found no difference between the volume of antigens given to kids with and without autism.

The study, published Friday in the Journal of Pediatrics, looked at 256 kids with autism and 752 without the developmental disorder born between 1994 and 1999.

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“These results indicate that parental concerns that their children are receiving too many vaccines in the first two years of life or too many vaccines at a single doctor visit are not supported in terms of an increased risk of autism,” wrote Frank DeStefano of the CDC and his colleagues in the report.

Scientists have previously debunked any link between vaccines and autism and the original study that sparked fears was retracted in 2010. Nonetheless, surveys continue to find that many parents are concerned, with about 1 in 10 refusing or delaying vaccination.

Most recently, many parents have turned their attention to the rigor of today’s recommended immunization schedule for children, arguing that young kids get too many vaccines too quickly.

Researchers in the latest study acknowledge that children today receive more vaccines than kids did even in the late 1990s. But, due to advances in immunology, kids actually receive far fewer antigens, they said. Children currently may be exposed to as many as 315 antigens by age 2 from vaccines, a figure that reached several thousand just two decades ago.

“This study adds to the existing epidemiological studies showing no link between vaccines and autism,” said Geri Dawson, chief science officer at Autism Speaks, who was not involved in study. “This research is very important for addressing the concerns of families.”