ews that President-elect Donald Trump and Robert F. Kennedy Jr. met on Tuesday rang alarm bells throughout the scientific community, because both have aggressively promoted discredited anti-vaccination theories.

Here are the debunked theories they espouse, and what the science actually says.

THE CLAIM: That the high number of vaccines given to young children can overwhelm their immune systems and cause autism, and that spacing out vaccines over a longer period of time would be safer.


THE SCIENCE: From the second a baby starts to descend the birth canal, it encounters multitudes of bacteria and other microbes to which its immune system must respond. It’s a process that goes on throughout life.

Just by breathing in dust or putting a dirty finger in a mouth, young children are activating their immune systems all the time.

The full complement of vaccines children receive in their first few years of life contain far fewer components that stimulate the immune response than they encounter by eating, drinking, and exploring their worlds, said Dr. Paul Offit, a vaccine expert and pediatrician at Children’s Hospital of Philadelphia. Offit is the co-inventor of a rotavirus vaccine.

Moreover, modern production methods actually produce more purified vaccines, so even though they’re getting more vaccinations, children today are exposed to fewer immunogenic components than they were decades ago, when they got just one vaccination.

“So although we have 14 [childhood] vaccines today … the fact is that we have fewer immunological challenges [from vaccines] than we did 100 years ago,” Offit said.

(By the way, that one vaccine routinely given decades ago prevented infection with smallpox. The world no longer worries about smallpox because it was eradicated — thanks to the vaccine.)

Despite these evident public health gains, skeptics have circulated a theory that childhood vaccines overtax the immune system in ways that can lead to neurological damage. People made nervous by that idea sometimes embrace an alternative vaccine schedule which recommends spacing vaccines out.

But this alternative schedule is not based on science — and experts say it’s actually dangerous, because it extends the period of time during which children are not fully protected against diseases that can cause serious illness and can even kill.

A 2010 study that compared children who got their vaccines on time to children who were vaccinated on a delayed scheduled or who did not get vaccinations showed no neurological benefit of spacing out vaccination.

And a 2014 study found that children who were vaccinated on a delayed schedule actually had higher rates of post-vaccine seizures, for certain vaccines, than children vaccinated on time. (Some people who believe their children were harmed by vaccines have talked about post-vaccination seizures and changes in behavior.)

THE CLAIM: That exposure to a preservative called thimerosal, used in some vaccines, is linked to rising rates of autism.

“If you actually read the science, which most journalists don’t do, the science is overwhelming that there is a link between the autism and the thimerosal.” — Robert F. Kennedy Jr. on “The Daily Show,” 2005.

THE SCIENCE: Before we get to the science, let’s consider a couple of crucial facts. Thimerosal hasn’t been used in childhood vaccines in the US for more than 15 years. And it was never used in the measles-mumps-rubella, or MMR, vaccine that some vaccine opponents have linked to autism.

Thimerosal is used in some vaccines for adults as a preservative, and before 2001 it was used in some childhood vaccines too. When used, it is added to vaccines that are sold in multi-dose vials because it kills bacteria. The preservative ensures that vaccine in a vial doesn’t become contaminated after the first dose is extracted.

Thimerosal is mercury-based, a type called ethylmercury. The human body breaks this substance down and clears it faster than methylmercury, the neuro-toxic substance that can be found in some fish and some paints.

Thimerosal became a focus of the anti-vaccination forces a number of years ago, when activists claimed it damaged young brains and caused autism. Study after study disputed that claim. But out of an abundance of caution it was eventually taken out of virtually all vaccines given to young children. (It is used in injectable flu vaccine packaged in multi-dose vials.) The decision was taken in 1999; childhood vaccines have been thimerosal-free since 2001.

If thimerosal had been causing autism, that development should have triggered a decline in autism cases. But autism rates did not fall in the wake of the withdrawal of thimerosal.

A study from California, published in 2008, looked at autism rates among children in that state before and after thimerosal was removed from vaccines. Rates went up after 2001. Denmark had taken thimerosal out of childhood vaccines earlier; it did not see a decline in autism rates, either.

Dr. Eric Fombonne, a child psychiatrist at Oregon Health and Science University and an autism epidemiologist, said the question about thimerosal has been thoroughly debunked. “These studies have been done with enormous cohorts of children, in different countries, by different investigators with different designs,” Fombonne said. “There was never a signal that was found.”

Andrew Joseph contributed to this report.

Clarification: This story was updated to clarify thimerosal is used in injectable flu vaccine packaged in multi-dose vials.

Sign up for Morning Rounds

A daily dose of health and medicine news — and a finalist for Digiday’s best email newsletter.

Recommended Stories