Parents fight every day to keep their children safe. They read nutrition labels on the foods they buy, search for cleansers that don't emit toxic vapors, and carefully read car-seat instructions before installation. For many parents, it makes sense to apply the same scrutiny to vaccines.

Unfortunately, finding accurate information isn't always easy. Social media sites are filled with self-proclaimed experts who insist vaccine ingredients cause autism. Many bloggers agree.

But is it true?

Worried parents can relax. Experts agree that vaccines do not cause autism. They know that the ingredients inside vaccines, including the elements bloggers love to attack, do not pose autism risks.

Do Thimerosal or Mercury Cause Autism?

Some vaccines are sold in vials filled with many doses. Doctors stick needles in the top, pull out the fluid, and apply the shots. In vaccines like this, thimerosal is added to keep bacteria and fungi from growing. Contamination is harmful; thimerosal is not.

Thimerosal is a mercury-based preservative, and it's been used in vaccines for years. Mercury is a toxin, and some people believe that it raises autism risks.

Since 2003, the Centers for Disease Control and Prevention has funded or conducted nine separate studies that all found no link between thimerosal and autism. Each study was handled professionally and carefully, and the conclusion was always the same.

The U.S. Food and Drug Administration shares several important thimerosal points:

  • Use is declining. Manufacturers are shifting from multi-dose formulations to single-use packaging. Since this type of vaccine faces fewer contamination risks, thimerosal isn't required.
  • Parents have choices. All vaccines come in thimerosal-free versions.
  • Preservatives are helpful. Microbial growth causes serious illness. Including preservatives can keep people safer.
  • Thimerosal was removed from vaccines to reduce overall mercury exposure risks. Researchers know this ingredient in a vaccine can't cause autism. But children are exposed to mercury throughout the environment. Taking this ingredient out of vaccines is one small, helpful way to reduce a child's overall mercury intake.

Does Formaldehyde Cause Autism?

Vaccine manufacturers must keep their plants sterile and safe. Bacteria can sneak into even the cleanest places, and it can thrive within the closed environment of a syringe. Formaldehyde can kill viruses and inactivate toxins, and it's included in some vaccines.

The amount we take in through a vaccine is tiny. Researchers say, for example, that the quantity of formaldehyde in a standard DTaP dose for infants is less than 0.005 mg to 0.1 mg. Meanwhile, most two-month-old children have about 1.1 mg of formaldehyde circulating in their bloodstream at any time due to environmental exposure.

Experts say we're exposed to formaldehyde every day via these sources:

  • Antihistamines
  • Car exhaust
  • Carpets
  • Cosmetics
  • Cough drops
  • Felt-tip markers
  • Mouthwash
  • Paint
  • Upholstery

There's no published link between formaldehyde and autism. But if such a connection exists, the child is at greater risk from crawling on the carpet in a standard home than from getting a vaccine.

Does Aluminum Cause Autism?

In a perfect world, a vaccine causes changes from the moment it hits the bloodstream. In reality, some vaccines need a kickstart. Adjuvants like aluminum help strengthen the body's response to vaccines, and they've been used since the 1930s.

Experts say many vaccines don't contain aluminum, but those that do contain amounts as low as 0.125 mg per dose. The average person takes in up to 50 mg of aluminum through drinking water and taking medication.

If aluminum caused autism, and there's no evidence that's true, the risk posed by the vaccine is much lower than the risk a child faces from drinking a glass of cool water.

Organizations Weigh In

If the science about vaccines and autism is so clear-cut, why do people believe that the two are connected? And how many organizations speak up to tell the truth?

The problem began in 1998 with a faulty study published by a gastroenterologist. He concluded that autism and vaccines were linked, but problems with the study include:

  • Small sample size. Only 12 children were included.
  • Ethics issues. The researcher ordered invasive procedures on the children without approval
  • Financial consideration. The researcher accepted money from an anti-vaccine organization.
  • Poor data collection. The symptoms in children started long before the study. That fact was not disclosed.

The study was retracted, and the researcher lost his license, but copies of the work float freely on the internet. Every day, people read this work and grow convinced that vaccines are dangerous.

Plenty of organizations try to refute misinformation, including:

  • Autism Science Foundation. This organization says the results of published studies are clear, and that no connection between autism and vaccines exists.
  • American Academy of Pediatrics. AAP works closely with the Centers for Disease Control and Prevention on recommended vaccines. No link between autism and childhood vaccines exists, the organization says, and it's not safer to space out vaccines than to give them all at once.
  • National Academies of Sciences, Engineering, and Medicine. A claim that vaccines cause autism is called out as false by this organization. No credible studies that prove the link exist, officials say.

What Really Causes Autism?

If vaccines don't cause autism, what's really causing an increase in autism prevalence? If we can't blame ingredients, where should we point our fingers as we look for a solution?

Researchers admit that autism prevalence is rising. Look at the numbers, without examining the underlying data, and it seems like we're in an epidemic that involves thousands of children. But match the data with information about autism diagnoses, and a new image comes to light.

What we call "autism" broadens every year. Diagnostic standards loosen, which means more children merit a diagnosis now, even though their symptoms are no different from those seen years ago. The children aren't different. The rule books are.

Autism Speaks says autism stems from a variety of sources, including:

  • Genetics. The disorder tends to run in families.
  • Environmental factors. Exposure to chemicals in utero could influence risk.
  • Parental health. Advanced age, pregnancy complications, or close pregnancies could all raise risks, while prenatal vitamins could lower them.

All of these influences, working together, could raise or lower a child's risk of impairment. One thing alone can't do the work. Everything is connected in a tangle that researchers haven't unwound quite yet.

If you're a parent, it's wise to watch the research and learn all you can about protecting your child. But for now, you can stop worrying about the influence of vaccines.


Thimerosal in Vaccines. (October 2015). Centers for Disease Control and Prevention.

Vaccines Do Not Cause Autism. (March 2020). Centers for Disease Control and Prevention.

Thimerosal and Vaccines. (February 2018). U.S. Food and Drug Administration.

What's in Vaccines? (August 2019). Centers for Disease Control and Prevention.

Vaccines: The Myths and the Facts. (August 2019). American Academy of Allergy, Asthma, and Immunology.

Vaccine Ingredients: Formaldehyde. (May 2018). Children's Hospital of Philadelphia.

Adjuvants Help Vaccines Work Better. (October 2018). Centers for Disease Control and Prevention.

Do Vaccines Cause Autism? (April 2019). Institute for Vaccine Safety, Johns Hopkins Bloomberg School of Public Health.

Autism and Vaccines. Autism Science Foundation.

Vaccine Safety: Examine the Evidence. (July 2018). American Academy of Pediatrics.

Vaccines Do Not Cause Autism. (October 2019). National Academy of Sciences.

Clear Answers and Smart Advice About Your Baby's Shots. Immunization Action Coalition.

What Causes Autism? Autism Speaks.

The Quest for Autism's Causes, and What It Reveals About All of Us. (February 2019). Knowable Magazine.