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Home » What causes autism? Maybe infections during pregnancy but not vaccines

What causes autism? Maybe infections during pregnancy but not vaccines

Despite the rigid beliefs of the anti-vaccine religion, the science is settled – vaccines are not causally linked to autism or any other neurodevelopmental issue. Arguing about this is a waste of time, if not for the fact that some parents might resist vaccinating their children because they heard these unfounded claims. But that doesn’t mean scientific research has quit looking for what causes autism.

Scientific research has focused on genetics as the underlying cause of autism spectrum disorder (ASD) (see Note 1). But often, environmental triggers could induce the upregulation of genes, which may lead to ASD.

During this search for what causes autism, scientists have uncovered a potential link between infections during pregnancy and autism. In fact, this new data may be evidence for pregnant women to be fully vaccinated before and during pregnancy to reduce the risk of those infections.

What is autism spectrum disorder

I’m not going to go into all the details about ASD diagnosis, symptoms and treatments – that would take up a book. But, I will spend a bit of time defining what constitutes autism spectrum disorder. ASD is a range of different conditions classified as neurodevelopmental disorders in DSM-5. However, individuals diagnosed with ASD must present with two general symptoms:

  • Deficits in social interaction and communication
  • Repetitive and restricted patterns of behavior interests or activities

Autism spectrum disorder encompasses diagnoses of autism, Asperger syndromea pervasive developmental disorder not otherwise specified (PDD-NOS), and childhood disintegrative disorder. All of these disorders include the following features – social deficits, communications difficulties, repetitive behaviors and interests, sensory issues, and, occasionally, cognitive delays.

Furthermore, there is a range of symptoms from minor to severe, with each person with ASD expressing it in an almost infinite variety of symptoms and severity. Although ASD is considered “incurable,” it is not a sentence of a mental prison for the patient. The lay public tends to box ASD children into one small stereotype, like people do with most mental health disorders, despite the fact that there is huge variability in how a child expresses ASD.

There is a variety of evidence-based interventions for autistic children. Many of the methods employ a psychoeducational approach to enhance cognitive, communication, behavioral, and social skills. There is no one single treatment that is best for all children with ASD, so treatment is typically adjusted to meet the needs of the child.

Although evidence-based interventions for autistic children vary in their methods, many adopt a psychoeducational approach to enhancing cognitive, communication, and social skills while minimizing problem behaviors. It has been argued that no single treatment is best and treatment is typically tailored to the child’s needs.

I think the most important thing to note about ASD is that it is a complex disorder with a wide range of symptoms. It’s not a “death sentence” for either the parents or child, despite what you may have heard on the internet.

Infection during pregnancy causes autism – a paper

In a paper recently published in Nature’s Molecular Psychiatry, M. Hornig et al. examined whether there was a link between maternal exposure to second-trimester fever and autism spectrum disorder risk. What they found was that a pregnant woman who had fevers during her second-trimester was 40% more likely to have a child born with ASD.

Using a prospective cohort study, the researchers followed a group of 95,000 children born in Norway over time, who differ with respect to maternal infections, and attempted to determine if those infections may have caused autism in the children. This study was done in Norway, where medical records are thorough for all citizens of the country, allowing researchers to pinpoint a correlation between a cause and an effect.

The relationship between maternal fevers and ASD seems to increase substantially with three or more maternal fevers, with a 312% increase in the risk of ASD children than mothers without an infection. And fever-reducing medications, such as acetaminophen (known as paracetamol outside of the USA, and is widely marketed as Tylenol), seem to reduce the risk for ASD. The researchers aren’t certain that these medications are a solution, since there other risks to pregnant women when they take them.

There is very strong evidence that a rubella infection during pregnancy leads to a significantly increased risk of autism in her baby. Immunization against rubella before or during may be an effective strategy to reduce the risk of autism in children of those mothers.

The evidence of a relationship between other viruses and autism is not quite as strong. For example, there is some very weak evidence (reviewed here) that influenza in pregnant mothers may be related to an increased risk of autism in their children. Since the flu vaccine is very safe for use in pregnant mothers, it is a good argument to get the vaccine.

Since you know I’m an advocate of biological plausibility to support a claim of causality, the authors presented a plausible explanation for their observations. During infections, the immune system releases inflammatory chemicals, especially pro-inflammatory cytokines. These cytokines can cross the placental barrier and have an effect on the developing fetus.

The authors concluded that:

Our findings support a role for gestational maternal infection and innate immune responses to infection in the pathogenesis of at least some cases of ASD.


This is pretty powerful evidence from a large study that infections (especially multiple infections) that induce a fever is strongly associated with autism spectrum disorder. And preventing those types of infections can lead to outcomes that reduce the risk of ASD. This is also powerful evidence supporting maternal flu vaccinations since flu infections can lead to fevers that last several days, which could increase the risk of ASD even further.

So we keep looking for what causes autism – and what we know is that it is not vaccines. But it could be maternal fevers from infection.


  1. I know that some people do not like the term “autism spectrum disorder” because it implies that autistic individuals have some terrible disease that makes them seem to be different than neurotypical individuals. However, science is generally unbiased, and “disorder” is a medical term that does not have the same meaning in medicine as it does to the general public. It’s ironic that medicine settled on the term “disorder” to be more value-neutral than “disease” or “illness.” I don’t think it is necessary to “purify” science of terms that have distinct medical and scientific meanings, so I’ll use ASD without implying any bias whatsoever.


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