As federal health officials offer more evidence that the mercury-containing vaccine preservative thimerosal is safe, many vaccine experts say in retrospect that the U.S. Food and Drug Administration's decision to remove it from childhood vaccines may have done more harm than good by raising public fears.
And still others argue that research and funds still being spent on exploring the risks of thimerosal could be directed to more productive enterprises.
A new study published in the New England Journal of Medicine concludes that early exposure to thimerosal does not cause any neurological problems. Thimerosal, used in vaccines since the 1930s, has been a topic of controversy since the FDA banned it in 1999.
Some claim that the additive causes autism and other brain development disorders in children. But the latest study joins a growing body of literature that shows thimerosal is safe and causes no long-term negative effects on children's health.
Although no concrete evidence at the time showed that thimerosal was harmful, the Centers for Disease Control and Prevention and the American Academy of Pediatrics pushed for its elimination to quell the fears of parents who might otherwise not get their children vaccinated.
But in an editorial published alongside this new research, Dr. Paul Offit, chief of the division of infectious diseases at the Children's Hospital in Philadelphia, argues that this move was likely unnecessary -- and it could have ended up causing even more alarm among parents.
"Critics wondered how removing something that hadn't been found to be unsafe could make vaccines even safer," Offit writes.
Several vaccination experts agree.
"Thimerosal was removed from vaccines as a preventive measure, even though there was no indication that it represented a health risk," Mark Slifka of the Vaccine and Gene Therapy Institute and the Oregon Health and Sciences University says.
"However, it is easy for a skeptic to see this change in policy and jump to the conclusion that because thimerosal was removed, there must have been something wrong with it. The conspiracy theorists will continue to claim that thimerosal removal is 'proof' that a health danger exists and the negative impact of these individuals will be difficult to reconcile."
"The removal of thimerosal created the impression of risk, where none existed," says Dr. Paul Krogstad, professor in the departments of pediatrics and medical pharmacology at the David Geffen School of Medicine at UCLA. "This well-intended effort sent a mixed message, and we will be facing the repercussions for years to come."
Still, others say the widespread fear over thimerosal forced regulators into a corner.
"The decisions were made in a setting of less information than we now have, and thus the precautionary steps felt prudent and responsible at the time," says Dr. John Modlin, chairman of the department of pediatrics at Dartmouth Medical School.