There’s no evidence that mercury-based chemicals in a Covid-19 vaccine would cause harm

Our readers have asked us to check claims that new Covid-19 vaccines contain enough mercury to cause brain damage.It is likely this question refers to thiomersal, a mercury-based chemical, but this is no longer used in most standard vaccines in the UK, Europe or the US. Even if it was used in a new Covid-19 vaccine, there is no evidence that the amount that would be in it could cause any harm. 

Thiomersal

Thiomersal (or thimerosal in America) is a mercury-based preservative that is sometimes used in tiny quantities in multi-dose vaccines to prevent the growth of bacteria and fungi when the vaccine is opened. 

There are two types of mercury to which people may be exposed: methyl mercury and ethyl mercury. Methyl mercury can be toxic to people, but ethyl mercury, which is contained in thiomersal, is quickly eliminated from the body so it does not build up to reach harmful levels. 

The World Health Organization (WHO) says safety concerns about the use of thiomersal in vaccines has been predominantly based on data regarding exposure to methyl mercury, rather than ethyl mercury, leading to initiatives to remove it from vaccines. 

It said: “It is important to note that concerns about toxicity of thiomersal are theoretical and that there is no compelling scientific evidence of a safety problem relating to its use in vaccines, although public perception of risk has been reported in some countries.”

Thiomersal was removed from most UK vaccines between 2003 and 2005, although it was used in the Swine Flu vaccine used in the 2009/10 and 2010/11 flu seasons. It is not currently found in any vaccines routinely used in the UK. 

The University of Oxford’s Vaccine Knowledge Project said it was removed from vaccines in the UK, Europe and the US as a “precaution” as part of a global goal of reducing exposure to mercury from all sources. However, it says there is “no evidence that thiomersal in vaccines caused harm”.

The US Centers for Disease Control and Prevention says most common side-effects of thiomersal are minor reactions like redness and swelling at the injection site. In 2014, an Australian study of over a million children found no evidence of a link between thiomersal in vaccines and autism development.

Will a Covid vaccine contain thiomersal? 

Although thiomersal is rarely used now, it is possible that one of the many Covid-19 vaccines currently in development will contain the preservative. 

As mentioned above, it was effectively used in the Swine Flu vaccine. The WHO still recommends use of vaccines containing thiomersal for global immunisation programmes “because the benefits of using such products far outweigh any theoretical risk of toxicity.”