The mercury-based chemical found in some UK vaccines is not dangerous

10 May 2019
What was claimed

Mercury is used as a preservative in vaccines and is not safe.

Our verdict

A mercury based chemical (and not the same one that can build up in the body after ingesting infected fish) is present in some non-standard UK vaccines. But it is not harmful in those amounts.

An image claiming vaccines contain mercury and implying they aren’t safe has been shared on Facebook.

Thiomersal is a mercury-based chemical no longer found in most standard UK vaccines. It was present in the Swine Flu vaccine used in the 2009/10 and 2010/11 flu seasons in the UK, but it’s not in any of the annual flu vaccines currently being used in the UK.

Thiomersal doesn't have the same chemical structure as the mercury that can contaminate fish and build up in the body. That is methyl mercury, which can cause major health issues. It also isn’t the same chemical structure as the mercury which was historically used in the hat making process—which the original Facebook post alludes to. That was mercuric nitrate which famously caused health issues for those coming into close, regular contact with the substance.

Thiomersal is an ethyl mercury, and exposure to it in vaccines is much lower than to methyl mercury in contaminated fish eaten regularly.

According to the Vaccine Knowledge Project at the University of Oxford (a project by a group of consultants, researchers, nurses and statisticians looking at vaccines at the university’s Department of Paediatrics) thiomersal was removed from most standard vaccines in the UK, the US and Europe “as a precaution”, and there was no evidence it caused harm. It was removed from UK vaccines between 2003 and 2005.

It’s correct that thiomersal (also known as thimerosal in the US) has been used in vaccines as a preservative, stopping the growth of bacteria and fungi. This is usually only in multi-dose vials, when an individual vaccine dose is drawn with a needle from the same container, which contains several doses.

According to the US National Institute of Allergy and Infectious Diseases the only known side effects of receiving low doses of thiomersal in vaccines are “minor reactions such as redness and swelling at the injection site”.

Several studies have shown infant and adult blood levels of mercury return to normal soon after being given vaccinations containing thiomersal. A 2014 study from Australia looking at over a million children found there was no link between thiomersal in vaccines and developing autism.

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