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.
"Saying or doing nothing might have provoked more or different suits," says Dr. Joseph Zanga, assistant dean for generalist programs at the Brody School of Medicine at East Carolina University in North Carolina. "This was a no-win situation engendered by our society's desire to have the perfect child, perfectly cared for. Most do not realize that we cannot protect children from all the dangers inherent in life, and by trying to do so we protect some while putting others at risk."
A Legal Mess
Most often, these repercussions have taken the form of legal battles. In his editorial, Offit describes how 4,800 parents of autistic children have taken their message to federal the Vaccination Injury Compensation Program.
And some say there could be more legal battles to come.
"There will be more, and more complex suits as we continue to develop more vaccines promising a life free of disease," Zanga says.
"Consider, for example, the HPV vaccine. If we immunize all preteen girls years before most are sexually active, what happens when they have their sexual debut after the vaccine protection has waned -- as most seem to these days, and the HPV vaccine is said to be protective for five to seven years?"
Others note that continuing to fund research to prove the safety of thimerosal instead of using funds for more productive purposes, such as determining the real cause of autism and how to cure it, is counterproductive.
"I am saddened that many families with autistic children continue to chase false causes when ... we should be placing our energies and resources into researching the real cause(s)," says Dr. Michael Muszynski, dean and professor of Clinical Sciences at the Florida State University College of Medicine.
In the most recent study funded by the CDC, researchers evaluated more than 1,000 kids between the ages of 7 and 10 who were exposed to various levels of thimerosal as babies.
Researchers tested the children extensively, assessing the children in 42 different areas of neurological functioning and making almost 400 different statistical comparisons. They found that for the vast majority of tests, children with high levels of thimerosal exposure performed equally well compared to children with low exposure levels, indicating that thimerosal has no effect on brain development.
"On the whole, the results are very reassuring," said Dr. Anne Schuchat, director of CDC's National Center for Immunization and Respiratory Diseases, in a press conference earlier today.
"The findings add weight to the preponderance of evidence that vaccines are safe and should be provided to infants and children," says Zanga. "The alternative, not immunizing and leaving the child at risk for disease, makes little general sense."
The study did find a small correlation between high thimerosal exposure levels and the presence of tics in boys. A couple of other studies have found similar results -- that tics, abnormal movements that look like twitches, are associated with high thimerosal exposure. The CDC says it will devote further research to the issue.
But Slifka says the weight of the evidence backs the safety of vaccines.
"This is yet more concrete proof that children's vaccines are safe," he says. "It is similar to multiple studies performed by highly reputable independent laboratories in several nations; there is no link between vaccines and autism."