Children who receive the measles, mumps and rubella (MMR) vaccine are not at increased risk for autism, and that includes children who are sometimes considered to be in "high risk" groups for the neurodevelopmental disorder, a massive new study finds.

The new study, published today (March 4) in the journal Annals of Internal Medicine, is one of the largest studies of its kind to date. In it, researchers looked at the records of more than 657,000 children born in Denmark between 1999 and 2010, including about 6,500 who had received an autism spectrum disorder (ASD) diagnosis. ASD is a neurodevelopmental condition that affects a person's ability to communicate, interact and behave appropriately with others in social situations.

The study shows, as many before it have time and again, that "[caregivers] shouldn't choose to not vaccinate because of this punitive association between the MMR [vaccine] and autism," said study principal investigator Anders Hviid, a senior researcher in the Department of Epidemiology Research at Statens Serum Institut in Copenhagen. "There's really strong science that there is no association." [Beyond Vaccines: 5 Things That Might Really Cause Autism]

The idea that the measles component of the MMR vaccine might be linked to autism began with a small, now-retracted 1998 study in the journal The Lancet. That research looked at 12 children with developmental delays, and eight of the kids had autism. It's since come to light that the lead researcher had several conflicts of interest: He had been paid by a law firm that wanted to sue the vaccine manufacturer, and he had a patent for a "safer" measles vaccine that he had developed before doing the 1998 study, according to a 2011 report in the journal The BMJ.

Since 1998, countless studies have found no link between the MMR vaccine and autism, including a large 2002 study in The New England Journal of Medicine that Hviid carried out with his colleagues; that research looked at 537,000 children born in Denmark between 1991 and 1998. But after the publication of that study, Hviid heard from concerned parents and so-called anti-vaxxers who questioned whether "susceptible" children might be at risk for autism after receiving the MMR vaccine.

"We saw an opportunity to re-examine the association in the same setting but with new children," Hviid told Live Science. "We also looked at how we could address some of the criticisms of our original study."

What they studied

In the new study, in addition to looking at the big picture (whether the MMR vaccine increases autism risk in all children), the researchers looked at whether the vaccine increased risk in the following groups: boys, girls, children who develop "regressive autism" when they're older and children whose siblings have autism (the condition is partly genetic, so these children already have a greater risk of developing autism than the general public does).

The scientists also looked at individuals' birth years, whether other childhood vaccines were received and when, and each child's autism risk factors based on the child's disease risk score, the researchers reported in the study.

In the results, none of the subgroups that received the MMR vaccine showed any increased risk for autism, the researchers found. Interestingly, the vaccine was even associated with a slightly lower risk of autism in girls and in children born from 1999 to 2001, the researchers reported.

What increases autism risk?

It's still unclear what biological mechanisms cause autism. But the study did find which groups were at highest risk for autism: boys, children born more recently (from 2008 to 2010), children who had no early vaccinations and, as mentioned, those who had siblings with autism. Other risk factors included having older parents, a low birthweight, a preterm birth and a mother who smoked during pregnancy.

The study is a "well-conducted investigation" showing what other studies before it have: that getting the MMR vaccine does not increase a child's risk of autism, said Kristen Lyall, an assistant professor at the A.J. Drexel Autism Institute at Drexel University in Philadelphia, who was not involved in the study.

This research also makes "the important contribution that even among groups with increased susceptibility to autism, MMR vaccination is not associated with autism," Lyall told Live Science in an email.

In an editorial published alongside the study, Dr. Saad Omer, a professor at the Emory Vaccine Center at Emory University in Atlanta, who was not involved with the study, said that the need to disprove anti-vaccine ideas comes at a cost. While large epidemiology studies may not cost as much as other types of research, he said, they do divert time that scientists could otherwise spend finding causes and treatments for autism.

"Irrespective of the absolute costs, the opportunity cost of this research should be kept in mind: For example, continuing to evaluate the MMR-autism hypothesis might come at the expense of not pursuing some of the more promising leads" related to autism's causes and treatments, Omer wrote in the editorial.

By Laura Geggel, Live Science