Study: Thimerosal in Vaccines Doesn't Negatively Affect the Brain

A decade ago, most moms and dads probably didn't know what was in their kids' vaccines. But one particular ingredient, thimerosal, has become a hot topic of conversation over the years, as researchers and parents alike ponder what effect, if any, the mercury-containing preservative has on children. And although immunizations for infants and young kids today contain no or very little thimerosal, the once widely used vaccine compound still remains highly controversial.

Although numerous studies have found no proven link, thimerosal has been blamed by some people as a possible cause of autism (a developmental disorder characterized by mild to severe impairment of communication and social interaction skills). Now, a new study for the Centers for Disease Control and Prevention (CDC) may offer parents further reassurance that thimerosal is not linked to negative effects on kids' brain developments — a message the CDC hopes will help parents feel more confident about the safety of vaccines in general.

It's important to note that this CDC study does not address autism specifically. (CDC researchers are conducting a separate study on thimerosal and autism, with results due to be released within the next year.) Instead, it looks at whether exposure to mercury from thimerosal-containing vaccines during pregnancy and infancy affects kids' brains. The conclusion: Researchers say their findings do "not support a causal association" between thimerosal exposure early on and neurological and psychological problems later.

More on the Study

Researchers gave 1,000+ 7- to 10-year-olds 42 different standardized tests designed to measure areas such as intelligence, speech, language, motor skills, and behavior. They also talked to the children's parents and teachers, then compared their findings to each kid's level of exposure to thimerosal while in the womb, within the first month after birth, and up to age 7 months.

Though they found no overall risk of harm, the researchers did find one possible concern: Boys with higher thimerosal exposure were about twice as likely to have tics than boys who'd had the lowest exposure. (Tics are a type of disorder that involves sudden, repetitive movements or sounds that can be difficult to control.)

But the researchers point out that although the study's evaluators noted tics in some of the boys with higher thimerosal exposure, their parents hadn't reported noticing them. And, tics are often temporary and may not be a sign of a long-term problem. But because two earlier studies found similar tic results, the CDC says researchers will be looking into this possible link much more closely.

What This Means to You

Despite the lack of scientific evidence that it causes any harm, manufacturers began removing thimerosal from kids' vaccines in 1999 to reduce childhood exposure to mercury and other heavy metals.

Now, the flu vaccine is the only one used in kids 2 and under that contains any of the preservative. Although some of the flu vaccines do have thimerosal in them, most of those available for children have only trace amounts and are technically considered thimerosal-free, says the CDC.

Still, some parents may hesitate to have their kids vaccinated because they're worried about the risks and the possibility of serious reactions. Although some vaccines may cause mild reactions — like temporary fever and soreness around the shot site — serious reactions are very rare.

All in all, the risks of serious reactions to vaccinations are small compared with the health risks associated with the often-serious diseases they're intended to prevent. But if you have concerns about any vaccine for your child, don't hesitate to talk to your doctor. Ask about the benefits and risks of each vaccine and why they're so important for kids of all ages.

Source: "Early Thimerosal Exposure and Neuropsychological Outcomes at 7 to 10 Years," New England Journal of Medicine, Sept. 27, 2007.