Here’s What We Know: Addressing Questions and Concerns Around Thimerosal

The U.S. Department of Health and Human Services (HHS) continues to raise concerns around thimerosal, in use as a vaccine preservative since the 1930s.  

In what follows, Dr. Karen Acker, an assistant attending pediatrician with expertise in infectious diseases and assistant professor of clinical pediatrics at Weill Cornell Medicine, recounts the history of its use, its removal from certain childhood vaccines in 2001 and how that removal has had the unintended effect of intensifying public concerns around its safety. 

What is thimerosal, and how is it used? 

Thimerosal is a preservative used in some vaccines and pharmaceutical products to prevent their contamination by bacteria and fungi.  

“It contains ethylmercury, a form of mercury that’s different from methylmercury—the toxic form of mercury we worry about. Ethylmercury is processed in the body differently from methylmercury, and it is quickly metabolized as waste,” Dr. Acker says. In other words, ethylmercury is not associated with toxicity, and neither is thimerosal.  

Why was it removed from childhood vaccines? 

Thimerosal was a common component of vaccines until 2001, when it was removed from all childhood vaccines due to increased vaccine hesitancy, traced to concerns over the preservative. 

As Dr. Acker explains, it was not removed because of safety concerns among doctors and scientists. The American Academy of Pediatrics (AAP) opted to remove it because the risk of decreased vaccine uptake was greater than the benefits of thimerosal as a vaccine ingredient.  

“Unfortunately, the decision to remove it may have exacerbated public concerns, as it seemed to validate that there was a concern in the first place,” she says.  

Have earlier concerns come to pass? 

“No, concerns over thimerosal were never based on evidence. In fact, they have been thoroughly debunked. There is no link between thimerosal in vaccines and autism or developmental delays in children,” Dr. Acker explains.  

She’s quick to note that autism rates actually increased after thimerosal was removed from all childhood vaccines in 2001. 

What role have concerns about thimerosal played in stoking vaccine hesitancy in this country? 

Manufactured concerns around thimerosal have contributed to vaccine hesitancy and fueled anti-vaccine sentiment, she says.  

Which thimerosal-containing vaccines have been singled out by the HHS? 

The only vaccines administered in the U.S. that currently contain thimerosal are multi-dose flu vaccines. These constitute a small percentage of the flu vaccine supply. As single-dose vials and pre-filled syringes do not require a preservative, they are thimerosal-free.  

How long might it take to remove thimerosal from these multi-dose vaccines? 

The process to remove thimerosal from vaccines and convert to single-dose vials could take months to years.  

As well, requiring its removal from multi-dose vaccines could potentially lead to shortages. 

Is thimerosal and its supposed connection to autism a pretext for adopting a more general anti-vaccine stance? 

Yes, Dr. Acker says. “It appears that this is one of the main arguments being used to advance an anti-vaccine mindset.” 

Does thimerosal cause autism? 

Thimerosal does not cause autism or any other neurodevelopmental symptoms in children. Numerous studies conducted by the Centers for Disease Control and Prevention (CDC), the World Health Organization (WHO) and the National Institutes of Health (NIH) have demonstrated that there is no link between thimerosal and autism or other adverse outcomes.  

What are the dangers of questioning the value of thimerosal and the vaccines that contain it? 

By continually calling the benefits of thimerosal into question, the Department of Health and Human Services, its leadership and its staff are sowing doubt and increasing vaccine hesitancy.  

“Vaccines are hugely beneficial to the health of our country’s population,” Dr. Acker says, “preventing serious infections and saving lives.” 

Make an appointment with a Weill Cornell Medicine pediatrician here.