The idea that vaccines cause autism was debunked by scientists some time ago. Yet it won’t go away.
Health and Human Services Secretary Robert F. Kennedy Jr.’s agency, for example, is reportedly launching a new study to look at potential links between vaccines and autism. And vaccine hesitancy is again rearing its head, even as measles outbreaks proliferate across the country.
The skepticism is confounding given that scientists say numerous studies have already shown that the measles-mumps-rubella (MMR) vaccine unequivocally doesn’t cause autism. “We will learn absolutely nothing with another study,” says Helen Tager-Flusberg, director of the Center for Autism Research Excellence at Boston University. “We’ve ruled it out.”
So why do doubts linger? Partly it is because of a since-retracted 1998 study, which was based on fraudulent data from just 12 children, that gave rise to the vaccine-autism theory and continues to fuel conspiracy theories.
A bigger reason: We still don’t know what definitively causes autism. If we did, it would help convince people what doesn’t cause it.
In short, autism’s causes remain complex. That is an understandably unsatisfying answer to parents with children who have been diagnosed with autism spectrum disorder.
What we do know
So what do we know about what causes ASD, a neurodevelopmental disorder that encompasses a range of conditions resulting in varying degrees of challenges with social skills, speech and repetitive behaviors?
First, the number of children diagnosed with autism has been rising. About one in 36 U.S. 8-year-olds is diagnosed with ASD, up from one in 150 children in 2000. Researchers attribute the rise largely to better awareness and broader diagnostic criteria.
Doctors typically screen for ASD when babies are 18 months to 2 years old. So it isn’t surprising that some parents turn to a recent exposure—such as the MMR vaccine—as a culprit. The first dose of the MMR vaccine, after all, is administered around the age of 1.
Some skeptics have pointed to thimerosal, a mercury-containing preservative used in some vaccines. But the MMR vaccine never contained thimerosal and the preservative was taken out of childhood vaccines in the U.S. in 2001, according to the CDC.
More than two dozen studies have looked at the MMR vaccine and autism and not found a link. These include a massive 2019 study in the Annals of Internal Medicine, which looked at more than 600,000 children in Denmark. These studies have been done in different styles and different countries. All have reached the same conclusion.
Rather than vaccines, scientists say genetics plays the dominant role—both inherited traits and spontaneous mutations in early conception.
In other words, ASD is largely out of our control. More often than not, people with heritable genetic traits for autism don’t even know it.
Risk factors
The remaining risk comes from the impact of environmental factors that occur largely in utero while the fetus’s brain is developing. So the groundwork is laid before we are even born.
“It’s among the most heritable of neuropsychiatric disorders,” says Dr. Daniel Geschwind, a professor of neurology, psychiatry and human genetics at University of California, Los Angeles.
Separately, there are at least 200 genes known to be affected by spontaneous mutations that occur when a sperm or egg is dividing after conception and that are linked to autism, says Geschwind. Studies have linked advanced paternal age to a higher risk of autism. Some scientists speculate the sperm of older fathers might be more likely to have such spontaneous mutations.
Most of the genetic changes and environmental exposures linked to autism occur during the late first trimester and second trimester, says Geschwind. “It’s extremely unlikely that anything that occurs past birth or the immediate perinatal period is going to have an impact,” he says.
Maternal immune system
Judy Van de Water, a professor of medicine at the University of California, Davis, studies autism from the perspective of an immunologist with a focus on the maternal immune system during pregnancy.
Her lab developed a blood test for maternal autoantibodies—immune cells that mistakenly attack the body—that can cross the placenta and target fetal brain proteins.
She says about 20% of autism cases stem from these maternal autoantibodies. It remains unclear when the autoantibodies form or what, if anything, triggers them.
But, if they are present, studies show a woman has a 98% risk of having a child with autism. Van de Water has created a company to make a commercial product so women can get tested before pregnancy. She hopes to have this on the market within a year following validation studies. “It doesn’t rule out other causes of autism,” she notes.
In addition to advanced paternal age, studies have found older mothers also have a greater risk of having a child with autism. The risks of older parents are independent, but paternal age has a stronger impact.
Many of the other identified factors linked to an increased risk of autism relate to maternal health during pregnancy. Some infections during pregnancy, such as high fevers, may be a risk factor, as is taking certain medications like valproic acid, used to treat seizures and bipolar disorder. Preterm births are also a risk factor.
Genetic and environmental factors together most likely result in an increased risk of someone developing autism, says Heather Volk, a professor at Johns Hopkins University Bloomberg School of Public Health. She and other researchers have studied air-pollution exposure during pregnancy and found an association with autism.
Protective action
Part of the frustration with autism risk is there isn’t much we can do to control it. Staying healthy during pregnancy, however, can help minimize risk.
Janine LaSalle, a professor of microbiology and immunity at University of California, Davis, says there is evidence suggesting that maternal health risks—such as obesity, weight gained during pregnancy and gestational diabetes—can increase the risk of having a child with autism.
