Worried about mercury in vaccines? Here's the science behind 'thimerosal'

As the debate over mercury in vaccines reignite around the world, Indian health experts dismiss any link between thimerosal and developmental disorders such as autism

Ever heard of a mercury-based compound in your vaccine? It’s called ‘thimerosal’ — a preservative that keeps multi-dose vials free from bacterial or fungal contamination. Introduced in the 1930s, it contains a form of mercury called ‘ethylmercury’, which is different from ‘methylmercury’ which is the toxic type found in certain fish and industrial waste. The two are often confused by people, although they behave very differently in the body.

 

While thimerosal was once common in childhood vaccines globally, many countries, including the US, began phasing it out around the early 2000s due to rising public concern over mercury exposure, despite no strong evidence of harm. However, certain India’s vaccines, especially multi-dose ones, are believed to contain thimerosal.   

 

Why is thimerosal in the news?

    The debate over thimerosal surfaced after some public figures made controversial claims linking mercury in vaccines to autism.

 

At the first meeting of US Health and Human Services Secretary Robert F Kennedy Jr’s special committee on vaccines, the panel voted to stop recommending flu vaccines that contain thimerosal. Public health experts warn that this action could undermine trust and spread confusion about vaccine safety.

 

Moreover, several studies that once claimed a link between thimerosal and autism have since been retracted or discredited.   

 

Does Thimerosal cause autism?

    According to a large body of international research, there is no credible evidence that thimerosal causes autism.

 

In 2004, the US Institute of Medicine reviewed over 200 scientific studies and found no link between thimerosal in vaccines and autism. Later reviews by the CDC (Centers for Disease Control and Prevention) and FDA (Food and Drug Administration) also found no evidence that thimerosal causes autism or developmental delays.

 

“Thimerosal, an ethyl mercury compound, preserves multi-dose vaccines by inhibiting microbial growth after repeated vial entry. Clinically, it reduces contamination risk in mass immunisation. Decades of surveillance and evidence show no link to autism or serious toxicity at approved doses. In fact, autism rates did not decrease after thimerosal was removed from vaccines, which supports that the vaccines do not increase the risk of autism,” said Dr Deepak Ugra, paediatrician at Lilavati Hospital, Mumbai.   

 

Is thimerosal still used in Indian vaccines?

    Many of India’s vaccines, particularly those supplied in multi-dose formats, are believed to contain thimerosal.

 

Dr Ugra explained, “Thimerosal remains in use within India’s routine immunisation programme, especially in multi-dose vials of vaccines like DTP and Hepatitis B. It serves as an antimicrobial preservative. While single-dose, thimerosal-free alternatives are available, their broader rollout is constrained by cost, infrastructure, and supply-chain limitations in public health settings. However, in the private sector only single-dose, preservative-free vaccines are used.” 

 

Is thimerosal safe for children?

 

Thimerosal breaks down into ethylmercury, which is eliminated from the body quickly and does not accumulate in the body. This sets it apart from methylmercury, which can be toxic in high amounts.

 

“Parents should not be concerned about mercury in vaccines. Ethyl mercury, one of the metabolites of thimerosal, is excreted quickly and does not have the neurotoxic properties of methylmercury. While mild reactions at the injection site such as redness, swelling, or tenderness, and rarely, contact hypersensitivity may be seen, serious adverse events are extremely rare,” said Dr Ugra.

 

While thimerosal remains a topic of public interest, current scientific evidence does not support claims linking it to autism. For parents in India, the key lies in staying informed through verified medical sources and consulting paediatricians for vaccine safety.      

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This report is for informational purposes only and is not a substitute for professional medical advice.