“Thimerosal is a mercury-based preservative that can help vaccines last longer,” Gandhi explained.

That is precisely what renders it controversial, since, as the World Health Organization (WHO) notes, mercury is a liquid metal that poses various risks to human health.

It can affect a number of bodily systems, including the nervous system, digestive system, immune system, the lungs, kidneys, skin, and eyes.

Furthermore, “high levels of mercury can be toxic to a developing fetus, for eample,” Gandhi pointed out.

For this reason, she added, “thimerosal has been taken out of most childhood vaccines but can still be found in some including the multidose influenza vaccine.”

However, thimerosal contains a very specific, modified form of mercury called ethylmercury, which, according to the WHO, “does not pose a health risk,” as it quickly breaks down after entering the body, and is subsequently eliminated.

Another part of the thimerosal controversy is the fact that it was mistakenly linked to autism by a now-discredited and retracted study.

“Multiple studies have shown no safety concerns with thimerosal in terms of causing autism or neurodevelopmental issues,” said Gandhi.

The advisory panel calling for thimerosal’s ban appeared to further fuel the controversy by citing a publication whose alleged author has since explained in media interviews does not exist, leading to its removal from the presentation.

“[T]his reference —showing an association between thimerosal and neurodevelopmental delays in animals — did not exist and was subsequently removed from the presentation when the error was pointed out,” Gandhi noted.

Speaking to Reuters, Katherine L. O’Brien, MD, MPH, director of the Department of Immunization, Vaccines, and Biologicals at the WHO, emphasized that, based on current medical evidence, the organization finds thimerosal safe.

Is thimerosal considered safe?

“Thimerosal has been reviewed multiple times by multiple agencies, including WHO, and it’s clear from the evidence that there is no evidence of harm from the use of thimerosal.”
— Katherine L. O’Brien, MD, MPH

While it is largely considered safe, in the U.S., thimerosal is already rarely used in vaccines. In 1999, the American Academy of Pediatrics and the U.S. Public Health Service issued a call to remove thimerosal from most childhood vaccines.

This ingredient has thus been avoided, as a precautionary measure, since 2001, according to the CDC.

The only exception to this, as Gandhi pointed out, is the multidose flu shot.

“Before we knew thimerosal was safe in multiple animal and human studies, it was removed from almost all childhood vaccines. However, thimerosal is still in influenza vaccine, especially the ones that come in multidose vials, since the agent serves as powerful preservative protecting the vaccine from bacterial or fungal contamination.”

— Monica Gandhi, MD, MPH

The greatest concern with banning thimerosal entirely is that this action could severely affect vaccine availability in the community.

“Banning thimerosal is simply not indicated by the scientific evidence,” Gandhi insisted.

“Moreover, this could result in less stock of influenza vaccine being produced or a move away from the multidose vials, which could limit vaccine availability of the flu vaccine this coming respiratory season,” she told us.

According to national immunization data provided by the CDC, flu vaccinations among children in the U.S. are already on the decline, even as flu cases were surging significantly during the last flu season.

Should these shots become less available during the next flu season, this viral infection, which is typically easily kept at bay, could cause more severe issues among the young.

According to the CDC, the flu can cause serious complications in young children, including pneumonia and inflammation of the brain (encephalitis).