Thimerosal in Vaccines: What Are the Facts?

Paul A. Offit, MD


December 28, 2012

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Hi. My name is Paul Offit. I am talking to you today from the Vaccination Education Center at Children's Hospital of Philadelphia.

Today is December 21, 2012, a day that according to the Mayan calendar should mark the end of the world as we know it. Speaking of ancient beliefs that aren't founded on good science, I thought it would be of interest to talk about a paper that appeared in the journal Pediatrics[1]this week, about thimerosal in vaccines.

Many of you might wonder why we are still talking about this. Hasn't this issue been resolved? Yes, it has, but it has come up again because of an effort by antivaccine groups that have lobbied the World Health Organization and other global health groups to try and get thimerosal out of vaccines given to infants and young children in the developing world -- something that would be disastrous.

In the late 1990s, as children began to receive more and more vaccines in the United States, they also received more and more thimerosal, an ethyl mercury-containing preservative in vaccines. Concern was expressed at the time that this may put children at risk. Mercury at high doses can cause harm, but the question was whether mercury in the form of ethyl mercury, given at much lower doses, could cause harm. This caused a great deal of concern in the late 1990s. As a consequence, there was a real effort to get thimerosal out of vaccines given to infants and very young children.

Since that time we have learned, in a series of 7 studies, that children who received thimerosal-containing vaccines compared with children who received the same vaccines without thimerosal are not at greater risk for neurodevelopmental problems, including autism or even subtle signs of mercury toxicity. In the late 1990s, a handful of children died of hepatitis B because the health centers in which they were born were so scared of thimerosal, which had been given a "scarlet letter," that they abandoned their hepatitis immunization program -- even for children who were born to mothers who had hepatitis B.

At the time, this action was considered a precaution: Let's get thimerosal out of vaccines until we learn more about thimerosal. Children died as a result; therefore, we didn't follow a precautionary principle that argues to do something to avoid harm, but in fact we caused harm. This new article is putting forward the idea that we should not make the same mistake, because now we know that the level of thimerosal in vaccines doesn't cause harm. If it is decided by the World Health Organization or other global health agencies to remove thimerosal from vaccines, it would mean using single-dose vials instead of multidose vials, which makes vaccines much more expensive for countries that already can't afford them.

If we were to do this, instead of a handful of children dying, hundreds or thousands of children will die. This paper is saying, "Mea culpa; let's not make the same mistake again." Thank you very much for your attention and happy holidays.