Countless studies have shown no link between vaccines and an increased risk of autism spectrum disorders, and that includes getting the flu shot while pregnant. So why are parents and expecting parents still hesitant to vaccinate?

Concerns that vaccines may cause autism have been worrying parents since fraudulent research introduced the theory in the late 1990s—even amid mounting evidence that proves otherwise. In light of more new studies disputing the link between autism and vaccinations, here are some relieving answers to your most pressing questions.

Why Were Vaccines Linked to Autism?

In the late 1990s, some researchers raised concerns over the amount of thimerosal—a mercury-containing preservative—found in many children's vaccines. Although thimerosal had been used as an anti-contamination agent for decades, the diphtheria-tetanus-pertussis (DTaP) vaccination was the only thimerosal-containing shot recommended for infants and children until 1991.

The researchers hypothesized that, as more thimerosal-containing vaccines like hepatitis B and Hib were added to the recommended schedule, babies were receiving too much of the chemical in too short a timeframe, which could potentially impact brain development.

In a totally separate (but coincidental) issue around this time, another group of researchers lead by a British doctor named Andrew Wakefield theorized that children who received the measles-mumps-rubella (MMR) vaccine (which never did contain thimerosal) were more likely to develop autism than those who did not receive it. By January 2011, however, Dr. Wakefield's study was discredited by the British Medical Journal.

Today, scientists and experts are confident that vaccines play no role in the onset of this developmental disorder. “More than a dozen studies across researchers, study designs, and populations have all concluded that there’s no relation between vaccines and autism," says Matthew Daley, M.D., a pediatrician for Kaiser Permanente in Colorado and a researcher who studies vaccine topics. Read on to find out more about these studies.

Thimerosal and Autism: What's the Link?

Thimerosal was removed from most vaccines by 2001 (it's still found in some flu shots). That's because researchers worried that children were being exposed to too-high levels from receiving multiple vaccinations in a short timeframe. This decision was based on what levels were considered safe for methyl mercury—the kind in fish, which is structurally very different from the ethyl mercury found in thimerosal. Although scientists suspected that thimerosal was much safer than methyl mercury, they decided to remove it anyway, just to be super-careful.

However, a large study published in Archives of General Psychiatry in 2008 found that cases of autism continued to increase in California long after 2001, when thimerosal was removed from most vaccines. "If thimerosal in vaccines were causing autism, we'd expect that diagnoses of autism would decrease dramatically after the chemical was removed from vaccines," says Eric Fombonne, M.D., director of the psychiatry division at Montreal Children's Hospital and a member of the National Institutes of Health advisory board for autism research programs. "Not only did cases not decrease, but they continued to rise. That tells us that something else must be responsible for rising rates of autism in this country."

A series of many studies in other countries and populations drew similar conclusions. "Thimerosal was removed from vaccines in Canada in 1996 and in Denmark in 1992," says Dr. Fombonne. "Autism is still on the rise in those countries as well."

In 2004, both the World Health Organization and Institute of Medicine each concluded no link between autism rates and thimerosal exposure after examining the health records of hundreds of thousands of children. What's more, the CDC found no link between thimerosal and autism in nine different studies between 2003 and 2012.

Research published in the journal Pediatrics in 2008 shows that babies excrete thimerosal too quickly for it to build up to dangerous amounts. In the study, researchers tested the blood mercury levels of Argentinean babies after they received routine childhood vaccinations (thimerosal is still used as a vaccine preservative there). They found that infants expel thimerosal about 10 times faster than fish mercury—so rapidly that it can't accumulate in the body between vaccine doses.

"This study helps to debunk a crucial basis of the autism-vaccines theory, which held that babies were getting so many thimerosal-containing shots that the chemical would build up in the bloodstream and eventually cross over to the brain, where it could theoretically impact development," says study author Michael Pichichero, M.D., a professor of microbiology/immunology and pediatrics at the University of Rochester Medical Center. "But thimerosal leaves babies' bodies way too quickly for that to happen, which just adds more proof that this theory is extremely unlikely."

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The MMR Vaccine and Autism

Many people confuse the controversy over the vaccine for measles, mumps, and rubella (MMR) with that of thimerosal, but the two have always been totally separate issues. In fact, MMR vaccines have never even contained thimerosal.

The link between MMR and autism gained traction following the publication of a very small British study (it only included 12 kids) published in a British medical journal, The Lancet. The study was lead by Dr. Andrew Wakefield and it concluded that children developed autism soon after they received the MMR vaccine. The theory: The measles portion of the shot causes inflammation and infection of the intestines, which can then spread dangerous proteins to the brain, causing damage that may lead to autism.

When this study was first published, it launched a frenzied debate that resulted in bigger, better-designed studies that have all failed to find any link between MMR and autism. By early 2010, The Lancet retracted Dr. Wakefield's research and by January 2011, the British Medical Journal publicly denounced Dr. Wakefield's research as "fraudulent," saying that he had "falsified data" and tampered with his research results to give the MMR vaccine bad publicity. At the time of his study, Dr. Wakefield had been involved in a lawsuit against the manufacturers of the MMR vaccine and would have gained money for winning, making his research an obvious conflict of interest.

The latest study, published in the Annals of Internal Medicine in March 2019, examined 657,461 Danish children born between 1999 and 2010. A majority of participants (95 percent of them) were vaccinated against measles, mumps, and rubella—and 6,517 received an autism diagnosis. The study concluded that the MMR vaccine doesn't lead to autism, whether or not kids had any risk factors.

Researchers suspect that parents may mistakenly associate the MMR vaccine with autism because signs of autism first appear around 12 to 15 months, which is also when the vaccine is first administered.

The Flu Vaccine and Autism

While the majority of flu vaccines don't contain thimerosal nowadays, multidose vials may have trace amounts to prevent bacteria, fungi, and other germs from forming. "Introduction of bacteria and fungi has the potential to occur when a syringe needle enters a vial as a vaccine is being prepared for administration," according to the CDC. "Contamination by germs in a vaccine could cause severe local reactions, serious illness or death. In some vaccines, preservatives, including thimerosal, are added during the manufacturing process to prevent germ growth."

Parents can always choose thimerosal-free alternatives of the flu vaccine though, and experts assert that it's safe for kids. Indeed, the CDC recommends that everyone over 6 months of age receive a flu vaccine, with rare exceptions. Read more about the guidelines here.

Many studies—including one published in August 2020 in the Annals of Internal Medicine—have also confirmed that getting the flu shot while pregnant isn't related to an increased risk of autism spectrum disorders. Researchers at Karolinska Institutet in Sweden looked at data from Swedish health registers for newborns born between October 2009 and September 2010. Specifically, they focused on 40,000 infants whose mothers received the H1N1 (swine flu) vaccine and 29,000 infants whose mothers did not get vaccinated. After seven years, experts detected no difference in autism spectrum disorder rates between the two groups. About 1 percent of children exposed to the vaccine received an autism diagnosis, while 1.1 percent of non-exposed children did.

Getting the flu vaccine during pregnancy is especially important since expectant women have a higher chance of severe illness (such as pneumonia) from influenza. That's because pregnancy changes your heart, lungs, and immune system. The flu vaccine also helps protect newborns from influenza during their first several months of life.

Should My Kid Get Vaccinated?

Although the American Academy of Pediatrics, the Centers for Disease Control and Prevention, and many other reputable organizations agree that vaccines do not cause autism, there are still small but vocal groups who believe they do. And amid that conflicting information, some parents might opt not get their children vaccinated "just to be safe," because they worry about other possible reactions, or because of religious or other beliefs.

"But if you choose not to vaccinate your child, you are increasing his risk of contracting serious diseases that can lead to complications, hospitalization, and even death," says Dr. Fombonne. For example, after the MMR vaccine was first linked to autism in England, many parents stopped vaccinating their children—and several children died during a measles outbreak in Ireland soon afterward. And a recent measles outbreak in the United States has infected hundreds of people.

For all the major childhood vaccinations (hepatitis B, rotavirus, DTaP, Hib, pneumococcal, polio, flu, MMR, chickenpox, hepatitis A, meningococcal), most experts agree that the many, many benefits from getting vaccinated far outweigh any possible side effects or risks.