Vaccines and autism From BabyCenter
I've heard that a preservative in some vaccines can cause autism — what's going on?
A controversy is raging over this right now, with government and mainstream scientists on one side, and several small but vocal advocacy groups on the other. And many parents have been left feeling confused and frightened about their children's health.
The advocacy groups say that thimerosal, a preservative used in vaccines, is responsible for an alarming rise in rates of autism among children in the United States and around the world. Most scientists say that's not so.
Over the last decade, a number of major medical institutions have reviewed the evidence from the United States and abroad, and all have concluded that there's no link between autism and exposure to thimerosal. But some health activists challenge the validity of the existing science and assert that the U.S. government has conspired with vaccine manufacturers to cover up the truth about thimerosal and autism.
We may learn more soon. Several health institutions are continuing to conduct research on the risks of exposure to thimerosal. What's more, now that the preservative has been removed from all childhood vaccines in the United States (manufacturers stopped using it in 1999), everyone will be watching what happens to autism rates in this country over the next few years. There's been no sign of a slowdown yet.
Why was thimerosal added to vaccines in the first place?
Thimerosal has been used for 70 years as a preservative to inhibit the growth of bacteria and fungi in vaccines. Many vaccines are stored most efficiently in large multi-dose vials from which health workers must draw individual doses, leaving the vaccine vulnerable to contamination every time the rubber top is punctured by a new syringe. Several deadly incidents of contaminated vaccines in the 1920s prompted vaccine manufacturers to begin adding preservatives to all multi-dose vials of vaccines.
Thimerosal used to be one of the most widely used preservatives. Now that most vaccines no longer contain thimerosal, they have to be stored in individual dose vials or pre-filled syringes — a system that's more expensive for vaccine manufacturers.
Is it true that thimerosal contains mercury?
Thimerosal contains a mercury compound known as ethyl mercury. This is not the same as methyl mercury, found in high amounts in some fish. Methyl mercury accumulates in human tissue and, at certain levels, can impair cognitive development in young children — which is why the FDA now says that toddlers shouldn't eat too much fish, for example.
Ethyl mercury, on the other hand, hasn't been as well studied, so not much is known about the health implications or long-term effects of being exposed to it. But research conducted by the National Institute of Allergy and Infectious Diseases (NIAID) has established that the body eliminates ethyl mercury much more quickly than it does methyl mercury, so ethyl mercury doesn't accumulate in human tissue.
NIAID is continuing to study ethyl mercury and its impact on human health. According to mainstream research to date, the only known side effects of exposure to thimerosal in vaccines are minor reactions such as redness and swelling at the injection site in some patients.
Is it true that children were exposed to unsafe levels of mercury from thimerosal?
From the mid-1980s until 1999, as shots were added to the list of routine childhood immunizations, children in the United States were exposed to more and more thimerosal. Some versions of the diphtheria-tetanus-pertussis and Hib vaccines, as well as the hepatitis B and flu shots, contained the preservative.
In 1997 the FDA reviewed food and drugs containing mercury and found that some children may have been exposed to a cumulative dose of 187.5 micrograms (mcg) of ethyl mercury from all sources during the first six months of life. This amount exceeded the Environmental Protection Agency guidelines for methyl mercury exposure. (There are no federal safety standards for ethyl mercury.)
As a precautionary measure, the Public Health Service agencies, the American Academy of Pediatrics (AAP), and vaccine manufacturers agreed in 1999 that thimerosal should be reduced or eliminated in all childhood vaccines. Today, childhood vaccines contain no more than trace amounts of thimerosal, and children are exposed to a cumulative dose of less than 3 mcg of mercury from vaccines by the time they're 6 months old.
What's the evidence that thimerosal is linked to autism?
Some advocacy groups point to a handful of studies done in the late 1990s that purported to show that thimerosal triggered autism. These studies were conducted by Mark Geier, M.D., and his son David. Mark Geier, a geneticist by training and a former researcher at the National Institutes of Health (NIH), has served as a consultant and expert witness in support of claimants in a number of vaccine injury cases brought before the National Vaccine Injury Compensation Program, as well as civil cases.
According to the Geiers, children exposed to thimerosal in vaccines are six times as likely to have autism as unexposed children. They base their conclusions on their analysis of data obtained from the Vaccine Adverse Event Reporting System (VAERS), a U.S. government reporting system that compiles vaccine-related health complaints.
In a detailed critique of the Geiers' findings, the AAP explained the problem with relying on VAERS data — namely, that the system collects complaints but has no means of evaluating their legitimacy. "Health effects reported to VAERS as being associated with vaccines may represent true adverse events, coincidental occurrences, or mistakes in filing," the AAP said.
Experts at the AAP were also troubled by the Geiers methodology, arguing that the father and son didn't specify "how their data were generated, thus preventing accurate review of their methods and replication of their outcomes." The Institute of Medicine (IOM) found the Geiers' work to be full of methodological flaws and dismissed the results as "uninterpretable."
Other experts have questioned the Geiers' qualifications and disputed their findings. One court official, who presided over a vaccine injury case for which Mark Geier served as a professional witness, said his testimony was "not reliable or grounded in scientific methodology and procedure. His testimony is merely subjective belief and unsupported speculation." Geier has been similarly admonished in a number of other vaccine injury cases.
What's the evidence that thimerosal is not linked to autism?
Over the last decade, a number of major medical institutions have reviewed the evidence from the United States and abroad and concluded that there's no link between exposure to thimerosal and autism. Here are details from some of the most recent reports:
In 2003, researchers from the U.S. Centers for Disease Control examined data from the Vaccine Safety Datalink (VSD) Project, a database in which eight HMOs log their patients' vaccine records, including any adverse reactions. The authors analyzed the records of more than 120,000 children at two different HMOs and found no difference in autism rates among children exposed to various levels of thimerosal.
In 2004, the World Health Organization examined the records of more than 100,000 children in Great Britain and found no link between thimerosal exposure and increased risk for autism. In fact, the children who had been exposed to thimerosal had lower rates of developmental disorders than the children who hadn't.
Also in 2004, the IOM evaluated the latest research on the issue, including five major studies that examined the health records of hundreds of thousands of children in the United States, Britain, Denmark, and Sweden as well as the Geiers' studies. (The IOM is a division of the National Academy of Sciences, a prestigious independent body not affiliated with the U.S. government.) The IOM concluded: "The body of epidemiological evidence favors rejection of a causal relationship between thimerosal-containing vaccines and autism."
Experts who have looked closely at the data also point out that the rise in autism rates does not, in fact, correspond to an increase in exposure to thimerosal. In Great Britain, for example, the incidence of autism has risen dramatically since the 1980s. But only one vaccine (the DTP) administered in Britain contains thimerosal. All the other vaccines given there are thimerosal-free, and always have been.
Thus, rates of autism have multiplied in Britain while exposure to thimerosal in vaccines has remained constant. And a 2003 study of children in Denmark found that autism rates continued to rise there at the same rate as they did worldwide, even after the country stopped using thimerosal in vaccines in 1992.
Wasn't there some link between the MMR vaccine and autism?
In 1998, the British medical journal The Lancet published a study connecting the measles-mumps-rubella (MMR) vaccine with autism. Researchers noticed that eight of the 12 autistic children being studied had started showing symptoms of autism around the time they received their MMR shots, and hypothesized that the children were having a physical reaction to the vaccine.
It turned out to be just a coincidence, and the study has now been repudiated by several of the researchers and retracted by The Lancet. The study had nothing to do with thimerosal, which has never been used in the MMR vaccine, but people continue to confuse the two issues.
How can I tell if my child received vaccines that contained thimerosal?
If your child was vaccinated after the year 2001, it's unlikely that he was exposed to more than trace amounts of thimerosal. In 1999 the Food and Drug Administration (FDA), the National Institutes of Health (NIH), and the Centers for Disease Control (CDC), along with the American Academy of Pediatrics (AAP), asked vaccine manufacturers to reduce or eliminate the use of thimerosal in vaccines, and manufacturers took steps to comply.
Some doctors' offices continued to use existing stockpiles of vaccines containing thimerosal, but most experts believe they would have been used up by 2001 or 2002. Ask your child's doctor if you want to know for sure.
Isn't thimerosal still used in flu shots and some others?
Thimerosal is still used as a preservative in adult flu shots. Thimerosal-free formulations are available for infants, children, and pregnant women, but there's not always enough to meet demand, and doctors routinely run out of thimerosal-free supplies.
Also, some tetanus-diphtheria booster shots, which are given to children age 7 or older, contain thimerosal. Finally, thimerosal is still used in some childhood vaccinations in other countries, mostly in the developing world.
What are the risks of not immunizing my child?
The benefits of vaccinating your child far outweigh the risks — for your child and for your community as a whole. A certain percentage of children have adverse reactions to vaccines, but such incidents are rare, given the large number of children vaccinated each year.
Julia McMillan, a professor of pediatrics at Johns Hopkins University in Baltimore and a member of the American Academy of Pediatrics Committee on Infectious Diseases, likes to remind parents about the number of serious diseases now controlled or eliminated by vaccinations.
"Many parents today are too young to remember the toll these diseases took before vaccines were developed," she says. "Polio has not been seen in the United States for decades. Measles, which still kills children in Africa every day, has been virtually eliminated in the United States. Mumps, which can result in deafness and sterility, is now rare in the United States. Within two years of the introduction of the Hib shot, there has been a 60 percent reduction in incidents of bacterial meningitis. Our vaccination program has been one of the most successful health campaigns — in terms of saving lives — in history. But it will only continue to be successful if people have their children vaccinated."
If enough people decided not to be vaccinated, these illnesses could easily spread to epidemic proportions again. We know this is true because it has happened: When measles vaccination rates in the United States dropped in the late 1980s, more than 100,000 people came down with the disease and 120 died from it. In 1998, when immunization rates were back up, only 89 people became sick from measles and no one died.
Other diseases, such as polio and diphtheria, are still only a plane ride away. And even if you and your family never leave the country, lots of people do travel and they can unknowingly bring these diseases back with them. The more people in your community don't get vaccinated, the more quickly disease can spread throughout the population.
Where can I get more information on thimerosal and vaccine safety?
This is a complex issue, and given the widespread rumors and contradictory reports, it's not surprising that parents are alarmed and confused. Start by talking to your child's doctor. And if you'd like to do some reading on your own, there's no shortage of information available on the Web.
Here are Web sites for major U.S. government health organizations that offer information on thimerosal:
• Centers for Disease Control
• Food and Drug Administration
• National Institute of Allergy and Infectious Disease
