Thimerosal is a mercury-based preservative that is used in vaccines to prevent the growth of bacteria and fungus. It has gotten a lot of attention over the years, but especially since it was banned from most child vaccines in 2001.
Today, the only childhood vaccines that contain thimerosal in extremely low levels are one DTaP and one DTaP-Hib combination vaccine. There is a thimerosal-free flu vaccine option for children.
Vaccine manufacturers started using thimerosal as a preservative in the 1930s after several children died from contaminated vaccines.
Thimerosal is an ethylmercury, which is created when mercury comes into contact with certain bacteria. It prevents the growth of bacteria, but safely passes through the body of humans and does not accumulate.
In comparison, pregnant women must limit the amount of fish they eat because it can contain methylmercury, a different type of mercury that will accumulate in the body and cause toxic neurological side effects in a baby.
Thimerosal was banned after doctors raised concerns that babies may be less able to eliminate mercury from their systems, but follow-up studies showed that babies “excreted significant amounts of mercury in stool after thimerosal exposure,” meaning it was removed quickly.
At that time, babies got three vaccines (DTP, Hib, and HepB) and the amount of ethylmercury exceeded the government’s limit for methylmercury exposure. There was no set limit for ethylmercury, but thimerosal was banned from childhood vaccines as a precaution.
Furthermore, studies do not show a link between thimerosal vaccines and autism. Even after thimerosal was removed from childhood vaccines, autism rates continued to increase, which is the opposite of what would be expected if thimerosal caused autism.
Even so, concerns about the safety of thimerosal persist. A recent example came from Robert F. Kennedy, Jr., a vaccine activist who said on an April 20, 2017 appearance on “Tucker Carlson Tonight”:
“[Mercury] is the most potent neurotoxin known to man that is not radioactive. How can we inject that into a child? If you take that vaccine vial and break it, you have to dispose of that as hazardous waste. You have to evacuate the building.”
Thimerosal-based vaccines are hazardous medical wastes, but Mr. Kennedy’s claims were debunked by Snopes.com experts who determined there is not enough thimerosal to require an evacuation.