USA TODAY has updated this 2011 story about vaccine myths.
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.
Myth 1: Vaccines cause autism
Few medical myths have been debunked as thoroughly as this one.
More than 20 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 BMJ, formerly known as the British 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, the director of the MIT Graduate Program in Science Writing. 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."
Myth 2: Vaccines may contain dangerous chemicals
Concerns about toxins in vaccines have revolved around three chemicals: mercury, aluminum and formaldehyde.
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 of Baby 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 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.
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."
Myth 3: Children receive too many vaccines, overwhelming their immune systems
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.
Myth 4: It's safe to space out vaccinations
More parents are delaying vaccines to avoid giving their children several shots at once, sometimes because they're afraid of inflicting unnecessary pain.
But spacing out vaccines may actually cause children more distress, Offit says.
Studies show that a child's stress hormone levels peak after one shot. Because that one shot is so stressful, giving a child additional needle sticks doesn't appreciably increase the child's distress, he says. So a child who receives one shot a month, instead of several at once, may actually have greater stress.
Postponing shots also leaves babies at risk, Brown says.
The vaccination schedule developed by the CDC is based on research to "protect as many babies as soon as possible," Brown says.
The "nasty little truth" to alternative schedules, on the other hand, is that they "are all fantasy," Brown says. None of the alternative schedules has been clinically tested — the kind of evidence upon which the CDC relies.
"There is absolutely no research that says delaying certain shots is safer," Brown says. "Doctors who promote these schedules are simply guessing when to give which shots. What we know for certain is that delaying your child's shots is playing Russian roulette."
Myth 5: Vaccines cause lots of serious side effects
Vaccines are incredibly safe, studies show.
Vaccines are tested in more children over a longer period of time than any other drug, Offit says. Research overwhelmingly shows them to be safe.
The human papillomavirus (HPV) vaccine, for example, was tested in 30,000 women before being approved, Offit says. The pneumococcal vaccine was tested in 40,000 children. The two rotavirus vaccines were tested in a total of 130,000 children. All were tested for more than 20 years.
When introducing any new vaccine, the FDA requires pharmaceutical companies to prove that their product doesn't pose a threat when added to the existing vaccine schedule, Offit says.
A special database, the U.S. Vaccine Adverse Event Reporting System (VAERS), helps scientists monitor vaccine safety, Offit says. Anyone can use the system to report a suspected side effect.
In many cases, the side effects reported to VAERS are coincidences.
Offit notes that 80% of people who reported to VAERS that vaccines caused autism were personal-injury lawyers, who may have a financial stake in making a claim.
Vaccine makers often take a cautious approach when writing their warning labels, listing all possible side effects, even if the side effects occurred at the same rate in unvaccinated people, Offit says.
Myth 6: Vaccine-preventable diseases aren't that dangerous
Vaccines are a victim of their own success, Mnookin says.
They have nearly eliminated diseases that once sickened, disabled or killed hundreds of thousands of Americans. Because few young parents have encountered any of these diseases, they don't realize how dangerous they are, he says.
Whooping cough, for example, once sickened 300,000 people a year and killed 7,000, mostly young children, Offit says.
Myth 7: The flu shot causes the flu
The viruses in the flu shot are dead, so they can't give anyone the flu, according to the Centers for Disease Control and Prevention. The most common side effect is soreness in the arm.
FluMist nasal spray contains weakened viruses, so they don't cause severe flu-like symptoms, either. They're specially engineered so they can reproduce in the nose — enough to stimulate the immune system to make antibodies — but those viruses cannot reproduce in the lungs and cause influenza, says William Schaffner, an infectious disease specialist who teaches at Vanderbilt University School of Medicine.
Side effects of FluMist in children can include a runny nose, wheezing and headache.
Flu shots tend to be given at a time of year when respiratory viruses are beginning to circulate, so it's not surprising that some people catch a respiratory bug shortly after getting a vaccine. And since it takes about two weeks after getting a vaccine for the body to develop immunity, some of those bugs could be the flu. But the vaccine itself isn't causing disease.
Myth 8: Only sick people need flu shots and other vaccines
When healthy people get vaccinated, it can help protect the weak, including cancer patients, anyone with a compromised immune system and newborns too young to get the shot, Offit says.
Because babies can't begin vaccinations against common disease until they're 2 months old, they depend on those around them — parents, grandparents, siblings, babysitters, hospital employees — to get the flu shot, creating a "cocoon" of protection, Schaffner says.
Myth 9: Vaccines contain tissue from aborted fetuses
Vaccines don't contain fetal tissue. The vaccine against rubella — also known as German measles — was created in the 1960s using a virus isolated from the tissue of fetuses whose mothers had rubella. The women chose abortion because of concerns about birth defects caused by rubella, according to the History of Vaccine website, a project of the College of Physicians of Philadelphia. Those birth defects include deafness, heart disease, mental retardation, a devastating brain inflammation called encephalitis and pneumonia.
Vaccines against hepatitis A, chickenpox and rabies also have been made with cell lines that derive from fetal tissue, Offit says. The viruses are purified before being used in vaccines, and no human cells remain in the final shots given to children.
The National Catholic Bioethics Center has said that Catholics are "morally free to use the vaccine, regardless of its historical association with abortion. The reason is that the risk to public health, if one chooses not to vaccinate, outweighs the legitimate concern about the origins of the vaccine. This is especially important for parents, who have a moral obligation to protect the life and health of their children and those around them. ... It is important to note that descendant cells are not the cells of the aborted child. They never, themselves, formed a part of the victim's body."
Myth 10: Vaccinated children can shed virus and infect others
Only one type of vaccine — the live polio vaccine, a liquid taken orally — has been shown to shed enough virus to affect other people, Offit says.
Children vaccinated with live polio vaccine can shed the virus through their feces and spread it to other kids if they don't wash their hands after using the bathroom. In the process, they can protect those kids against the virus.
Doctors call this phenomenon "contact immunity," and it was a useful trait in the 1950s, when the country was ravaged by polio, because vaccinating one child against polio could help to indirectly vaccinate others, Offit says. Rarely, however, live polio vaccines also shed in a way that causes polio.
By 2000, after polio had been eliminated in the USA, American doctors stopped using the live virus, because its risks outweighed its benefits. Since then, children have received only killed viruses in polio vaccines.
No other type of vaccine has ever been shown to shed, Offit says. So children vaccinated against measles won't spread measles to other kids by shedding the virus.
Measles can spread only when people are actually sick with the measles and showing symptoms, such as a cough, runny nose, fever or telltale red rash, says Gregory Wallace of the Centers for Disease Control and Prevention.
Correcting medical myths can be difficult, Schaffner says. While some parents welcome the information, others dig in their heels, remaining skeptical of information provided by government officials or pharmaceutical companies.
"We've reached a tipping point," Offit says. "Children are suffering and dying because some parents are more frightened by vaccines than by the diseases they prevent."