The anti-vaccination movement got a big boost in the 2000s thanks to scientific uncertainty and confusion about a vaccine additive.
The additive was thimerosal, a mercury-based preservative used for decades in multi-dose vaccines and medicines to prevent the growth of germs, according to the Centers for Disease Control and Prevention (CDC). Concern about cumulative mercury levels prompted the precautionary removal of thimerosal from childhood vaccines in 2001.
The tiny amounts of thimerosal in vaccines break down to ethylmercury, which leaves the body quickly and harmlessly, CDC says. Most vaccine experts at the time believed that was the case, but they did not have the scientific evidence needed to conclusively combat a growing controversy over thimerosal.
Congressional hearings, flawed scientific papers, and magazine articles focusing on thimerosal raised concerns that mercury in vaccines had caused autism in generations of U.S. children. Those fears were laid to rest in 2004 when numerous scientific studies showed thimerosal did no harm. But by then, vaccines and thimerosal already had received years of bad press.
Vaccine additives mostly work as adjuvants – substances that improve the patient’s immune response, says Alan Howell, MD, an infectious disease specialist in Temple and a member of the Texas Medical Association’s Committee on Infectious Diseases.
Like thimerosal, they also can work as preservatives. And some additives like sugar stabilize the vaccine to make sure it remains effective after it’s been produced, CDC says.
Many additives sound toxic, like aluminum and formaldehyde. Others sound strange, like squalene, which is made mostly from shark liver oil.
But vaccine additives pose little threat of harm for two reasons, Dr. Howell says. First, the dosage is too minute.
“On the surface, formaldehyde does sounds scary, and it could be dangerous under different circumstances,” he said. “But we’re talking about such small amounts.”
Second, these substances occur naturally. Most are already in our bodies and in our day-to-day environment, Dr. Howell says.
“In all likelihood, you’re being exposed to more of those chemicals in the course of your life by just going about your business,” he said.