My introduction to antivaccine thinking came six years ago, during a natural birth class. To our list of big decisions, the teacher added another — whether to go along with routine vaccinations, the first of which (for hepatitis B) is typically given before the baby leaves the hospital. • We chose the standard, doctor-recommended vaccine schedule for all three of our children. But for some people, childhood vaccination is a fraught issue. • At a recent child's birthday party, for example, one father mentioned that he and his wife had opted out of vaccinating their children. They didn't feel comfortable with the mercury in vaccines, apparently referring to suspicion of its link to autism.I was stumped. The preservative thimerosal, which contains a form of mercury called ethyl mercury, was removed from most vaccines in the United States more than a decade ago and has long since been absolved of causing autism.Thimerosal has been used as a preservative to prevent bacterial and fungal growth in certain multidose vaccines and other biological products since the 1930s. In the late 1990s, health officials began to grow increasingly aware of the dangers of mercury in fish and shellfish. The mercury found in fish is an entirely different form than what is in thimerosal. It's called methyl mercury; it grows more concentrated as it moves up the food chain and is neurotoxic in large amounts, especially to developing fetuses.•••But the seafood contamination issue led to concerns about the trace amount of mercury used in vaccines. "At that time — 1999 — there really wasn't sufficient scientific information to know about the differences between how ethyl mercury would act versus methyl mercury," says Peter Hotez, dean of the National School of Tropical Medicine at Baylor College of Medicine in Texas. Still, even then, enough was known about the genetic basis of autism and the unique clinical syndrome caused by mercury poisoning to make any causal link between autism and thimerosal highly unlikely, he adds.As a precaution, the Food and Drug Administration called for a review of the mercury content in food and drugs. Although there was no evidence of harm from the ethyl mercury in vaccines, the American Academy of Pediatrics and the U.S. Public Health Service acted pre-emptively to avoid any potential harm and to protect public trust; in 1999, these agencies issued a ban on the use of thimerosal in routine vaccinations given to infants and children. By the early 2000s, thimerosal had been removed from vaccines such as those for hepatitis B, diphtheria and tetanus toxoids and acellular pertussis (DTap), and haemophilus influenzae type B (Hib).The ban did not apply to annual influenza vaccines; U.S. health organizations concluded that the benefits of the influenza vaccines far outweighed the low risk of harm from thimerosal."There was never actually a problem with thimerosal," says Roger Baxter, co-director of the Kaiser Permanente Vaccine Study Center in Oakland, Calif. "It was all manufactured by rumor."•••Numerous studies have looked at whether children who received mercury-containing vaccines had poorer outcomes or increased risk of autism or other disorders, and no causal link has been found.Still, the controversy surrounding thimerosal helped buoy the antivaccine movement in the late '90s. About the same time, the movement began to focus on the measles-mumps-rubella (MMR) vaccine, spurred by a now-discredited study claiming it could cause autism. The paper was found to be fraudulent, and author Andrew Wakefield, a British doctor, lost his medical license.In recent years, antivaccine proponents have begun to focus on timing, suggesting that too many vaccines too early in life can overload a child's immune system and cause autism."The good news here is that these are questions that can be answered by science, and science has answered them," says Alison Singer, president of the Autism Science Foundation, an organization that helps fund research into the causes of autism. Pointing to the many papers that have explored and refuted the links between autism and thimerosal, the MMR vaccine and vaccine timing, Singer says: "They're all showing the same thing — that vaccines do not result in an increase in the diagnosis rate of autism. The question is, 'Are you as a parent willing to believe what the data clearly say?' And the majority of parents are." Singer's daughter, who is now 16, was diagnosed with autism at the age of 2.•••Nevertheless, it is easy to get your child fully vaccinated without exposure to a mercury-containing preservative, says Neal Halsey, director of the Institute for Vaccine Safety at the Johns Hopkins Bloomberg School of Public Health. For example, parents can request versions of the influenza vaccine that have no mercury, Halsey says.Overall, risks from vaccines are low, Halsey notes. About one per million doses can lead to a serious complication such as a severe allergic reaction."But it's at least a thousand times safer to give your child the vaccine than it is to let them be unvaccinated," he says.