(NEW YORK) -- In spite of all the anecdotes about autism-spectrum disorders and other neurological problems caused by vaccines, no scientific studies have shown a definitive link between the two. Yet many parents remain fearful. Why parents continue to question the safety of vaccines is the subject of an article published in PLoS Biology, a peer-reviewed, open-access journal published by the Public Library of Science.
The article focuses on the research of a University of California-San Francisco medical anthropologist, Sharon Kaufman, PhD, who got interested in the persistent doubt around vaccines after reading reports of scientists, doctors, and government spokespeople receiving harassing phone calls and even death threats for simply reporting on findings that vaccines don't cause autism.
Much of the doubt was triggered by a 1998 British study published in The Lancet. It reported on a theory that the measles, mumps, and rubella (MMR) vaccine caused intestinal problems that released toxic substances into the brain. The paper was later discredited, and the author is currently under investigation for ethics violations.
Around the same time, a congressman in the U.S. asked the Food and Drug Administration to review the use of thimerosal (a mercury-containing preservative) in all the products it regulates. The agency found that its current use could expose children under 6 months old to dangerously high levels, and as a result, asked pharmaceutical companies to remove it from vaccines. Since 2001, thimerosal preservatives have been largely eliminated from childhood vaccines (the flu vaccine being the one exception), although it's still detectable in trace amounts as a by-product of manufacturing.
Those two events were enough to generate serious doubt in parents' minds about the safety of vaccines, even though medical agencies around the world continued to publish papers and reviews disproving theories of a vaccine-autism link. Increasing access to the Internet didn't help either, Kaufman found, as collecting that information online led to more doubt.
According to a 2002 review of anti-vaccination websites published in the Archives of Disease in Childhood, many sites used emotional appeals and cited seemingly convincing evidence that came from self-published studies (which don't undergo the peer review process required by medical journals).
The researchers also found that it wasn't uncommon for the sites to publish statistics that came from questionable sources, including letters to newspaper editors and television interviews, rather than scientific research. Also, the review found, many of these sites promoted the idea of vaccinations as a "hoax" intended to generate profits for doctors and pharmaceutical companies.
"Despite all of the very hard work of epidemiologists to exonerate vaccines as a cause or to be implicated in autism, the lay public still seems to have rampant suspicion about what vaccines are used for and why we use them," says Dr. Joseph Domachowske, a fellow of the American Academy of Pediatrics and professor of pediatrics, microbiology and immunology at the State University of New York Upstate Medical University. "Until we can give people an answer as to what causes autism, there will still be some suspicion about vaccines."And the suspicion is causing concerned parents to lose sight of the bigger picture.
"People have forgotten about the infections that vaccines can prevent," he says. "And there's a misconception that vaccines are riskier than developing a vaccine-preventable infection." Those infections are on the rise in many parts of the world. Measles, one of the deadliest childhood illnesses, was all but eliminated in the U.S. before 2000 but last year saw the largest outbreak since then. And just this week, officials in Europe announced that cases of the mumps have doubled so far this year, compared with the same period last year.
Despite the scientific evidence, many parents still distrust childhood immunizations' safety. But why?
In general, says Dr. Domachowske, a majority of parents do vaccinate their children, but "they seem to want to know a whole lot more than in the past before they agree to it," he adds. If you need more information to put your mind at ease, though, rely on doctors and epidemiologists rather than emotional blogs that don't often rely on experts, he says.
Even though most childhood vaccines no longer contain mercury-based preservatives, you can ask for thimerosal-free flu vaccines (the flu shot is the only one administered to children under 7 that still contains thimerosal). Dr. Domachowske notes that trace amounts of thimerosal left over from manufacturing expose children to very tiny amounts mercury, 1 microgram (mcg) or less per dose, according to the FDA. In comparison, exclusively breastfed infants will ingest 400 mcg of mercury by the time they're 6 months of age, just from normal environmental exposures to the mother (mercury in air and water) that pass into her breast milk.
According to a recent survey, only 9 percent of toddlers are up-to-date on their immunizations, but falling behind could make the vaccines less effective. A common concern expressed on anti-vaccine websites is that the one-size-fits-all vaccination schedule, which is set by the CDC, the American Academy of Pediatrics, and the American Academy of Family Physicians, doesn't take into account children's differing health needs. Talk to your doctor about your child's medical history if you're concerned that a vaccine may interact with an existing allergy or a past infection.
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