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Thimerosal is a preservative that has been used in drug products since the 1930s

Hundreds of studies have shown that the mercury-based product is safe for humans

(CNN) —  

Thimerosal is best known as a preservative used in some vaccines to keep them from becoming contaminated.

The preservative has gotten a lot of attention over the years, particularly since it was removed from childhood vaccines in 2001.

Several studies have shown that the preservative is safe, but not everyone has been convinced. Here’s what you need to know about what it is (and what it isn’t).

What is thimerosal?

It is the most widely used preservative in vials of vaccines used multiple times, a mercury-based organic compound that can prevent bacteria and fungus from growing.

The antiseptic is also used in some drug products.

Why do vaccines need preservatives?

Vaccine makers started using preservatives in the 1930s after they found that contamination could become a problem with multi-dose vaccines. Doctors learned that the hard way in 1928, when 12 children died after getting vaccinated for diphtheria.

An investigation found that the multi-dose vaccine had been contaminated with living staphylococci. The children had been injected with the diphtheria vaccine and a staph infection.

Scientific committees recommended that vaccine makers use preservatives, and some did. It wasn’t until 1968 that the United States started requiring manufacturers to use them in most multi-use vaccines.

What happens to the preservative in your body?

Your body easily eliminates the thimerosal. Unlike chemicals that might stay in your body for a long time, it is quickly removed from the blood and excreted in your waste.

Thimerosal does not build up in your system like other mercury-based compounds can.

Is it safe?

Hundreds of studies have shown that it is extremely safe for humans. Several comprehensive reviews have shown there is no evidence of harm caused by low doses.

In animals, some studies have shown central nervous problems, coma and death, although the same has not been found in humans.

Are there any side effects?

The most common side effect is a mild rash or redness at the injection site. There may also be a little swelling. All of these symptoms disappear quickly.

On rare occasions, some people have had allergic reactions to the preservative.

Do children get vaccines with thimerosal?

All routinely recommended vaccines for children in the US are available in a thimerosal-free formulation or contain only a trace amount.

Thimerosal was taken out of most of the vaccines young children get in 2001, with the exception of the flu vaccine, which contains small amounts. You can ask your doctor for a flu shot that is thimerosal-free.

Some vaccines – including measles, mumps and rubella; chicken pox; polio; and pneumonia – were never made with thimerosal.

What makes it different from other mercury-based products?

Thimerosal is an ethylmercury; the mercury that can be found in fish is a methylmercury. Though they are only one letter apart, the substances are different.

Mercury is an element found in the Earth’s crust. We are all exposed to mercury as we live on the surface of the planet. It’s in our water, in our soil and in the air.

Methylmercury is created when mercury comes into contact with some bacteria. Doctors warn pregnant women to avoid some fish because it can be contaminated with this form.

Unlike ethylmercury, which can pass through your body quickly, methylmercury can linger and accumulate. If enough accumulates, it can be toxic.

In 1999, the National Vaccine Advisory Committee held a meeting to discuss the safety of thimerosal. Some who testified worried that babies were unable to eliminate the mercury from their systems. Followup studies of infants showed that they “excreted significant amounts of mercury in stool after thimerosal exposure,” meaning it was removed from the body quickly, compared with methylmercury.

If it is safe,