Many people are asking questions about thimerosal, a mercury-based compound that was once used in vaccines. While thimerosal used to be common in childhood vaccines, it has now been mostly removed. Still, it is sometimes used in a few adult vaccines. Let’s break down what it is, why it was used, and whether it is safe.
What Is Thimerosal?
Thimerosal is a preservative that was first used in vaccines in the 1930s. It helps keep vaccines safe by preventing bacteria and fungi from growing inside the vials. About 50% of the weight of thimerosal comes from a form of mercury called ethylmercury.
Many people are concerned when they hear the word “mercury” because they know that mercury can be toxic. For example, eating too much tuna, which can contain mercury, is unsafe. But it’s important to know that not all mercury is the same. Some forms of mercury are dangerous, but others, like ethylmercury, are cleared from the body quickly.
Understanding the Different Types of Mercury
There are different forms of mercury in our environment. It can come from volcanoes, forest fires, coal burning, and factories. Some forms of mercury, such as methylmercury, stay in the body for a long time and build up. This is the type found in fish like tuna and swordfish. If consumed in large amounts, it can cause brain problems.
Ethylmercury, the type found in thimerosal, is different. It stays in the body for a very short time — just 3 to 7 days — and is quickly eliminated. Scientists say the amount used in vaccines is very small and not harmful. In fact, a person would need to get hundreds of vaccines at once to have this problem..
Why Was Thimerosal Removed from Childhood Vaccines?
In the late 1990s, the U.S. government decided to reduce people’s exposure to mercury. A law was passed that listed all foods and drugs that contained mercury. At the time, thimerosal was found in three childhood vaccines. Although there was no strong evidence that thimerosal caused harm, doctors wanted to be more careful.
By 2001, thimerosal had been removed from almost all childhood vaccines. The only exceptions are some adult flu vaccines that are stored in multi-dose bottles. They still require preservatives to prevent contamination.
Now, more than 96 percent of all flu vaccines in the U.S. are thimerosal-free. Vaccines that do contain thimerosal contain very small amounts — less than 1 microgram per dose — far below the safety limits set by health agencies.
What Science Says About Thimerosal Today
Since thimerosal was removed, scientists have conducted many studies to learn more. More than a dozen studies have found that thimerosal does not cause autism or other brain problems. In 2004, a top U.S. medical organization said that vaccines containing thimerosal are safe.
Although medical experts agree that thimerosal is not dangerous, its removal helped to ease public concern. Some doctors say that removing it before the research was complete helped maintain public confidence. Others feel that it has made people worry unnecessarily.
Why It Matters Today
Daily news highlights show that vaccines are still a hot topic, especially during flu seasons and health emergencies. This week, U.S. health officials are re-evaluating vaccine safety, and thimerosal is part of the debate.
Although thimerosal is no longer used in vaccines for children, it is still important for people to understand why it was used and why scientists say it is safe. This knowledge helps combat misinformation and builds trust in science and vaccines.
