in light of the chaotic cc post on vaccines (more in comments, article too long )

Few medical myths have been debunked as thoroughly as this one. More parents have been skipping or delaying vaccinations for their children, a trend that has contributed to recent outbreaks of nearly forgotten diseases such as measles, mumps and whooping cough. Much of the anxiety about vaccines is based on myths or misinformation, infectious disease specialists say. Leading experts talked to USA TODAY's Liz Szabo to address some of the reasons parents hesitate to vaccinate their kids.

Fourteen scientific studies have failed to find a link between autism and vaccines, says Paul Offit, director of the Vaccine Education Center at Children's Hospital of Philadelphia.

The myth was fueled by a small, flawed study in The Lancet in 1998, which was later retracted. British medical authorities in 2010 found the author guilty of serious misconduct related to the study — including accepting more than $675,000 from a lawyer hoping to sue vaccine makers — and banned him from practicing medicine in England.

Editors of BMJBritish Medical Journal, called the study"an elaborate fraud," accusing author Andrew Wakefield of deliberately falsifying medical data.

In 2011, a U.S. "vaccine court" handling the Omnibus Autism Proceeding, in which judges considered lawsuits from roughly 5,000 families, ruling against parents who claimed that shots caused their children's autism.

But myths, once unleashed, can be hard to rein in, says Seth Mnookin, an assistant professor of science writing at the Massachusetts Institute of Technology. He wrote the book The Panic Virus: The True Story of the Vaccine-Autism Controversy.

"This idea has been set in people's minds, and it's going to take a while to overcome it," Mnookin says. "I talk to people who look at the research and say, 'I just don't trust it.' But for this to be a conspiracy, it would have to be virtually every government in the world."

Concerns about toxins in vaccines have revolved around three chemicals: mercury, aluminum and formaldehyde.

Mercury

Vaccines have never contained methyl mercury, the toxic metal that can cause brain damage, says Offit, who helped develop a vaccine against rotavirus, which kills 450,000 children a year.

Before 2001, some vaccines contained thimerosal, a preservative made with ethyl mercury. But ethyl mercury, which is safe, is very different from methyl mercury, which is toxic.

The difference is important, says obstetrician-gynecologist Jennifer Gunter, author of The Preemie Primer.Consider, she says, the huge difference between ethyl alcohol, which is drinking alcohol found in wine and beer, and methyl alcohol, also called wood alcohol, which can cause blindness.

Some parents have worried that thimerosal exposure could cause autism.

So is thimerosal harmful?

Not according to seven studies that have failed to find any link between thimerosal and autism.

To address parents' concerns, however, the Food and Drug Administration ordered that thimerosal be removed from routine childhood vaccinations. It hasn't been used since 2001.

Today, thimerosal is found in only one type of shot. Flu vaccine stored in multidose vials use the preservative to prevent the growth of fungus or other potentially dangerous germs, says Ari Brown, a spokeswoman for the American Academy of Pediatrics and author ofBaby 411.

Parents who remain concerned can ask for a thimerosal-free version, which is readily available. Neither flu shots in individual-dose containers nor the FluMist nasal spray contain thimerosal, Brown says.

Aluminum

Aluminum is used in small amounts in some vaccines to stimulate a better immune response, Offit says.

Yet babies get far more aluminum from food, including breast milk, than from vaccines. In the first six months of life, Offit says, a breast-fed baby takes in 10 milligrams of aluminum; a baby given a milk-based formula takes in 30 milligrams; a soy formula-fed baby gets 120 milligrams.

A baby who receives all recommended shots takes in only 4 milligrams of aluminum from them, he says.

Aluminum is also found in self-rising flour, Offit says. For many people, the biggest source of aluminum is cornbread.

Formaldehyde

Vaccines contain trace amounts of formaldehyde.

So do our bodies. Young infants have 10 times as much formaldehyde circulating in their bodies as is found in any vaccine, Offit says.

"If you have zero tolerance for mercury, you have to move to another planet," Offit says. "We all have mercury and formaldehyde and aluminum in our bodies. Vaccines don't add to what we normally encounter every day."

There's no sound evidence to support concerns, Offit says.

Two studies show no increase in autism among children who got several vaccines at an early age.

What many parents don't realize, he says, is that kids today get less of a challenge to the immune system from vaccines than their parents and grandparents did, even though kids today get more shots. A century ago, kids were vaccinated against only smallpox. Today's kids no longer get smallpox shots, but they're vaccinated against 14 other diseases.

Yet today's vaccines together contain fewer germ particles — the proteins that prime the immune system to respond to infections — than a single smallpox vaccine, Offit says.

The vaccine against smallpox — the largest of the world's more than 1 million viruses — contained 200 germ particles, Offit says. That's more germ particles than are found in all 14 of today's shots combined, Offit says.

The new shots also are engineered to be more targeted than earlier generations of vaccines, says Brown, the pediatrician.

For example, whooping cough shots made before 1991 contained 3,000 different germ particles, or antigens. Today's version has only three to five, Brown says.

Experts say the immune system is stronger than many people realize.

When leaving the womb, babies are immediately surrounded by millions of bacteria in the birth canal. If the immune system weren't so robust, humans wouldn't survive being born, Offit says