If Thimerosal Is Safe, Why Is It Being Removed From Vaccines?
There's a newly appointed panel of experts at the U.S. Centers for Disease Control and Prevention (CDC), freshly chosen by U.S. Health Secretary Robert F. Kennedy Jr. Called the Advisory Committee on Immunization Practices, it sets the immunization schedule for Americans. And some of the new members have histories of vaccine-skepticism.
On June 26, this panel voted to remove thimerosal from flu vaccines. The ingredient has long been the target of anti-vaccine activists, despite numerous studies showing it's safe in small amounts.
The committee's recommendation now goes to the CDC's acting director to become a formal recommendation. (Susan Monarez, President Trump's selection to head the agency, is currently undergoing confirmation hearings).
Here's how thimerosal got into vaccines, why it's being taken out, and what the latest recommendation could mean for next season's flu shots.
Thimerosal, which contains a form of mercury called ethylmercury, has been used in vaccines as a preservative since the 1930s to prevent growth of fungi and bacteria. At the time, most vaccines came in larger vials that contained multiple doses, and while the vaccine makers produced a sterile and safe product, 'each time you pierce a needle [into the vial] you are potentially introducing contaminants that can lead to infection,' says Dr. Jason Goldman, president of the American College of Physicians. 'While the vaccine itself was safe, the vials got contaminated, and there were deaths from infection. So the decision was made to put a preservative in it.'
That preservative—thimerosal—was used routinely in vaccines until 2001, when federal health officials decided to remove it from the majority of childhood vaccines. It is currently only used in flu vaccines that come in multi-dose vials. (Most of the flu vaccines in the U.S. come in single-dose syringes that do not contain thimerosal.)
Ethylmercury is cleared from the body more quickly than methylmercury, which is primarily found in deep-sea fish like tuna. Both types of mercury can be toxic to cells, particularly in the brain, but the more quickly-cleared ethylmercury has less time to cause such harm, according to the CDC.
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'Data from many studies show no evidence of harm caused by the low doses of thimerosal in vaccines,' the CDC's website says. That includes studies that looked at a variety of neurological and psychological outcomes, as well as autism. However, researchers say longer term data on the health effects of exposure to both types of mercury isn't completely clear yet.
In 2001, federal health officials decided to remove thimerosal from most childhood vaccines, which at the time included shots for influenza, diphtheria, tetanus, pertussis, and hepatitis B. Thimerosal remained in trace amounts in larger vials of the annual flu vaccine to protect against contamination.
Dr. Paul Offit, a member of the Advisory Committee on Immunization Practices at the time, says there were a number of reasons for the decision back then—most of which had more to do with policy pressures and optics than with concerns over health harms. In early 1998, Andrew Wakefield, a gastroenterologist in the U.K., had published a paper in which he linked the MMR vaccine for measles, mumps, and rubella to an increased risk of autism. His paper has since been debunked and his medical license has been revoked, but his findings sparked the anti-vaccine movement that continues today, despite evidence that vaccines have saved lives and provide more benefit than harm.
After Wakefield's paper gained notoriety, a U.S. Congressman asked the CDC's vaccine experts to review the data and vote whether, as Wakefield suggested, the vaccines for the three diseases should be separated and given individually to reduce any potential harm to infants. It wasn't based in science, and 'around the table we voted 'no,'' says Offit, who was part the committee.
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A few months later, vaccine maker Wyeth decided to remove RotaShield, the first vaccine to protect against rotavirus, from the market—just 10 months after it was approved. The vaccine was linked to rare bowel obstruction in some babies who had received it in their first six months. The withdrawal understandably added to the public's concerns about the safety of vaccines.
So when issues about the potential risks of mercury in thimerosal emerged, in part due to Wakefield's paper, Offit says the committee was faced with addressing Americans' growing concern that federal health agencies were not adequately ensuring vaccine safety. 'We had a vaccine that had been approved and then taken off the market; we did not buy Andrew Wakefield's notion to separate the MMR vaccine into three component parts. And now we had thimerosal,' says Offit. The committee's leader, as well as the members, 'were cognizant of the fact that it looked like were weren't paying attention to vaccine safety, or that we didn't care. Because we approved RotaShield, and we didn't listen to Andrew Wakefield.'
'My understanding was that at the time, there was essentially no evidence of any harm from thimerosal,' says Dr. Sean O'Leary, chair of the committee on infectious diseases at the American Academy of Pediatrics. 'But the concern was that it probably hasn't been studied as well as perhaps we'd like. And since we have the technology to remove it from the childhood vaccine schedule, we should go ahead and do that. But many, many people questioned that decision.'
One of them was Offit, who can't recall if there was an actual vote, but says he would have voted against removing thimerosal had there been one. Nothing about the makeup of the vaccines changed—only the formulation to package it into sterile, single-use syringes. 'It was an anti-science move,' he says. 'It did nothing to make vaccines safer—all we did was make them more expensive. We didn't explain ourselves. We didn't trust the American public to understand the nuance.'
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As a result, the decision to remove thimerosal was interpreted by many in the public, including anti-vaccine activists, as acknowledgement that thimerosal was unsafe and harmful. That decision, says Offit, 'gave birth to two anti vaccine groups—Moms Against Mercury and Generation Rescue. Any reasonable person would have thought, 'Why take it out so precipitously unless there was a problem?''
Because of that decision, currently only about 3% to 4% of flu vaccines in the U.S. that come in multi-dose vials contain thimerosal. And these larger vials are mostly used in rural and low-resource settings since they are less expensive than single-dose syringes. Most children who have received their immunizations at pediatrician offices for decades now have not been exposed to thimerosal. 'I don't know of any pediatric practices that use the multi-dose vials,' says O'Leary. 'It's pretty uncommon. Even if pediatricians did use the multi-dose vials, it's a non-issue because it's safe. This is a very clear effort to shine a light on this anti-vaccine trope that thimerosal is somehow dangerous.'
What concerns health experts about the new recommendation is that normal protocols governing the CDC panel's agenda and presentations weren't followed. Kennedy, a long-time vaccine skeptic, oversees the CDC and in June replaced all 17 previous members of the immunization committee with eight new members, many of whom lack expertise in vaccines and immunology. Dr. Cody Meissner, a faculty member at Dartmouth College Geisel School of Medicine, is the only pediatrician on the committee and also served on the U.S. Food and Drug Administration's expert vaccine committee. He was the only committee member to vote against the proposal to remove thimerosal from flu vaccines, telling other members that 'of all the issues that ACIP needs to focus on, this is not a big issue.'
'The real question is, why was this even brought up?' says Goldman. 'The thimerosal question has been asked and answered multiple times. We are not only re-litigating the issue, but now, instead of a fringe group refusing to accept evidence, they are now the decision makers.'
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The CDC committee includes liaison members of experts from professional organizations who add expertise and perspective to the discussions, but aren't voting members. Many such groups, including the American Academy of Pediatrics, decided not to attend the latest committee meeting in protest over the firing of the previous members.
"We made the decision that this was an illegitimate meeting because of the way the Secretary dismissed all 17 members who were highly qualified, and hand-picked people who are a mix of COVID-19 contrarians and outright anti-vaccine folks," says O'Leary. "We decided not to legitimize the process with our presence."
The experts TIME spoke to about the latest decision say that the CDC posted a description and references for studies that have supported the safety of thimerosal in vaccines, but that it was removed from the agency's website before the committee meeting. 'I am very concerned that this committee will do everything it can to undermine the vaccine schedule and the public's trust in science and data,' says Goldman. 'If that happens, and vaccines are no longer appropriately recommended, it will lead to an increase in infections and deaths and put the entire public health system at risk.'
The recommendation is 'unprecedented,' says O'Leary, and therefore it's not clear how long manufacturers will have to comply, or what happens to existing vials that are already purchased and stored for the upcoming flu season, since they are legally approved. It's also unclear how insurers will address the change, and whether they will continue to cover flu shots from multi-dose vials containing thimerosal, which are mostly used in lower-resource settings.
A spokesperson for Sanofi Pasteur, one of the larger flu vaccine makers in the U.S., said 'a very small number of our doses supplied in the U.S. contain thimerosal. We now await the decision by the CDC on the path forward.'
It's not clear yet what impact the recommendation, if adopted, will have on vaccine supply, although since the bulk of flu shots currently do not contain thimerosal, the number of available doses shouldn't be affected. Sanofi's spokesperson anticipated that 'we will have sufficient supply of Sanofi flu vaccine to support customer preference for this season.'
Still, since lower-resource and rural settings may rely on the less expensive multi-dose vials, in these areas, supply of the vaccines could dwindle if clinics can't afford to purchase as many of the single-dose syringes. 'The big concern from my perspective is that vaccinating the entire population for influenza every year is a herculean task,' says O'Leary. 'And there have been issues over the years because of shortages for one reason or another. To eliminate roughly 5% of the flu shot supply all of a sudden shortly before flu season to me means that fewer people may get vaccinated—and more hospitalizations and deaths.'
Contact us at letters@time.com.
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