An expert panel that advises the Centers for Disease Control and Prevention on vaccines voted Thursday to no longer recommend annual flu shots that contain the preservative thimerosal.
The move gives credence to the long debunked theory that the ingredient — which has been used in vaccines for nearly a century — is linked to neurodevelopmental problems. Medical experts said the decision also represents a jarring departure from the Advisory Committee on Immunization Practices history of making decisions based on strong scientific data.
“A.C.I.P. makes recommendations based on scientific evidence as much as possible,” said Dr. Cody Meissner, an expert in pediatric infectious disease epidemiology and voting member of the panel.
“There is no scientific evidence that thimerosal has caused a problem,” he added.
The vote followed a presentation from Lyn Redwood, a former leader of a prominent anti-vaccine group. Ms. Redwood perpetuated the idea that the mercury-based preservative is “neurotoxic” and harmful for pregnant women and children — claims that are contradicted by dozens of rigorous studies and have largely been rejected by vaccine scientists. Her presentation drew harsh criticism from the medical community, who questioned why a C.D.C. scientist did not lead the presentation, as is custom.
“Many statements made here today were without support of science and evidence but merely opinion,” said Dr. Jason Goldman, president of the American College of Physicians at the meeting.
The presentation came at the first committee meeting since the health secretary, Robert F. Kennedy Jr., fired all of the previous 17 members and appointed a controversial slate of new members, including several people with a track record speaking against vaccines.
Thimerosal has emerged as an early point of conflict between longtime vaccine scientists and the new panel of disrupters. A document posted on the C.D.C.’s website earlier this week offered a detailed breakdown of decades of research on thimerosal, concluding that the “evidence does not support an association” between the preservative and any neurodevelopmental disorder. That document, however, was quickly removed from the website.
How does this impact flu vaccines?
Thimerosal helps prevent vials that contain multiple shots from becoming contaminated when new needles draw out a dose.
It is rarely included in vaccines in the United States. The flu shot was the sole routinely recommended vaccine to include the preservative. And roughly 96 percent of all flu shots in the United States were thimerosal-free during the last influenza season, according to data from the C.D.C.
A representative from the Food and Drug Administration said that fully switching to single-dose vials would not limit availability of the vaccines in the United States during the upcoming flu season.
However, Dr. Meissner said he worried that a switch to single-dose vials, which are more expensive and difficult to store, would limit vaccine access globally.
“The recommendations the A.C.I.P. makes are followed by many countries around the world,” he said. “It’s going to reduce access to these vaccines and increase cost.”
What’s the controversy around thimerosal?
The ingredient was used in several childhood vaccines for decades with little concern.
In 1999, however, a sweeping review of mercury levels in food and drugs led to worries that vaccines containing thimerosal may expose children to unsafe levels of the heavy metal.
Experts are particularly attentive to mercury exposure in young children because high levels of exposure to heavy metals can affect their brains during a critical stage of development.
Several public health groups, including the American Academy of Pediatrics and the C.D.C., agreed to remove the ingredient from children’s vaccines as a “precautionary measure.”
The decision was controversial, said Dr. Paul Offit, a vaccine researcher at the Children’s Hospital of Philadelphia who was part of the A.C.I.P. review of thimerosal at the time. A faction of scientists, including Dr. Offit, believed the ingredient should not have been removed, since there was no credible evidence linking thimerosal to neurodevelopmental problems.
Others believed that until they had more evidence proving the additive was safe, it was a prudent choice to remove thimerosal. They also feared that if they left the ingredient, parents might shy away from vaccinating their children.
The decision to remove thimerosal did not quell concerns from anti-vaccine groups about the preservative. Instead, the decision seemed to validate their belief that it was dangerous, and the groups continued to push for a total thimerosal ban.
Mr. Kennedy was a leading voice in this charge, meeting with top health officials to push for the ban and writing high-profile articles about the dangers of the ingredient.
Is thimerosal safe?
While large amounts of mercury can be dangerous, humans are routinely exposed to small amounts in the soil, air and water.
“High levels is toxic, no doubt about it,” said Dr. Offit. “Is the mercury that we’re exposed to routinely toxic? The answer is no. If it was, we’d have to move to another planet.”
The amount in some flu vaccines is roughly equivalent to the amount of mercury found in a three-ounce can of tuna fish.
Furthermore, the type of the mercury used in certain vaccines, ethylmercury, is broken down and excreted by the body much faster than methylmercury, the form naturally found in the environment. That means mercury used in thimerosal is far less likely to build up in the body and reach toxic levels.
Since thimerosal was removed from childhood vaccines, scientists have published numerous well-designed studies that have found no credible link between the preservative and neurodevelopmental disorders, cognitive problems or autism.
For example, one study from Danish researchers analyzed the medical records of every child born in the country between 1990 and 1996 (in total, more than 460,000 children), to see whether those who received vaccines containing thimerosal had higher rates of autism than those who had been given vaccines without the preservative. They found no association. Dozens of similar studies have come to the same conclusion.
“In retrospect, obviously, we didn’t find any dangers,” said Dr. Walter Orenstein, an emeritus professor at Emory University who led the National Immunization Program at the C.D.C. during this decision.
And overall, autism rates did not decline after thimerosal was removed from the vaccines, as many anti-vaccine groups suggested it would. Instead, the rates increased.
Teddy Rosenbluth is a Times reporter covering health news, with a special focus on medical misinformation.
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