When it comes to the history of vaccines, it's been a long, bumpy ride. Once hailed as lifesaving wonders of modern technology, vaccines are now more likely to be a source of suspicion and angry playground debate.
Will we ever agree on the risk and benefits of vaccines? Probably not.
But to sort out fact from fiction, Health.com took a look at the scientific research to date on vaccines.
Some vaccines contain mercury
Thimerosal, a preservative containing about 50 percent mercury, prevents contamination by bacteria. It can be found in most flu shots, according to the Centers for Disease Control and Prevention (CDC).
However, since 2001, thimerosal has not been present in routine vaccines for children younger than 6. And, both the flu shot and some vaccines for adults and older children can be found in thimerosal-free versions, or with only trace amounts.
Vaccines cause autism
A small 1998 study by Andrew Wakefield claimed to find a link between the measles, mumps, and rubella (MMR) vaccine and autism, setting off a panic that led to dropping immunization rates, and subsequent outbreaks.
Since then, the study's been deemed flawed, and it's been retracted by the journal that published it. In 2004, the Institute of Medicine released a report that found no scientific evidence of a link between the MMR vaccine and autism. In September 2010, the CDC published similar results.
"It's more risky for your child to not be vaccinated," said Dr. Carrie Nelson, chair of the Commission on Health of the Public and Science for the American Academy of Family Physicians.
Vaccines can have side effects
Vaccines aren’t risk free. The most common side effects are soreness at the injection site and fever, which are best treated with acetaminophen or ibuprofen. Less common are seizures (defined as “jerking or staring”), and risks vary depending on the vaccine. For example, one in 14,000 children suffer a seizure after receiving the DTaP shot; it’s one in 3,000 with the MMR vaccine.
Some kids are at higher risk for side effects than others. In these cases, it may be best to proceed with caution or skip them, according the CDC.
You’re safe if everyone else is vaccinated
Unfortunately that’s a big if. "Often, like-minded unvaccinated families by choice attend the same preschools, playgroups, and schools, thus making it very easy for vaccine-preventable diseases to spread," said Dr. Ari Brown, MD, pediatrician and spokesperson for the American Academy of Pediatrics.
Dr. Brown says this was true with outbreaks in San Diego and Boulder, Col. And some people can’t be vaccinated due to health or age restrictions. Plus, you can catch some germs, like tetanus and hepatitis A, from contaminated soil or food, not another person.
Vaccines guarantee protection
Vaccines are not a 100 percent guarantee you won’t get sick. But they are a huge help.
Take the flu vaccine; you may still get the flu if you get the jab, but it is likely to be less severe. Or, take the chicken pox vaccine. Brown said it is 80 percent effective against preventing infection and 100 percent effective in protecting against serious illness.
For the best protection, experts rely on "herd immunity"—the more people who are vaccinated in the population, the better chances of protecting everyone, including people who can’t get shots due to age, health, or religious reasons.
Too many shots weaken the immune system
Brown said it’s quite the contrary. "Each dose allows the body to mount an immune response and make defense [antibodies] so the body can fight off a real infection if it showed up," she said.
Children are given multiple vaccinations at a time to provide as much protection as early as possible. Both the Advisory Committee on Immunization Practices and the American Academy of Pediatrics recommend that vaccinations be given to children simultaneously when appropriate.
Vaccines are for kids only
There are numerous vaccines that can help keep adolescents and adults, both young and old, healthy. Most obvious is the flu shot, which is given annually.
College students should get a meningitis vaccine before living in a dorm, and elderly adults can benefit from pneumonia vaccines.
Adults also need boosters for tetanus and pertussis. Children aren't fully immunized against pertussis until age 4, Nelson said; smaller babies are at high risk, and pertussis can be transmitted to babies by adults with waning immunity.
The HPV shot is for girls only
There are two HPV vaccines: Cervarix, for girls and women 10 to 25, and Gardasil, for females 9 to 26. But Gardasil can also be given to boys and men between ages 9 to 26, according to the CDC. Gardasil protects against types 6 and 11 of human papillomavirus, which cause about 90 percent of all genital warts.
Nelson said she recommends the HPV shot to patients, but "it’s variable. It's like a 50-50 split." Still, she says, girls definitely receive it more often than boys.
According to the CDC, approximately 500,000 cases of genital warts occur each year in the United States.
Pregnant women can’t get vaccines
Well, this is partially true. According to the American Academy of Family Physicians, pregnant women should not be given vaccines for varicella (chicken pox) or MMR.
But the inactivated flu vaccine is safe and even recommended for pregnant women, Brown said. During pregnancy, women's immune systems are compromised, making them more susceptible to infection.
But many are not getting the flu shot; the CDC says that, at last estimate, only 11% of pregnant women got one. Brown said the shot triggers the mother’s antibody production, protecting her baby through the first six months of life.
Natural immunity is better
Nelson said infections are more likely than vaccines to trigger lifelong immunity. (An exception is the flu; it changes strains every year.) But you may think twice about taking your little one to a chicken pox party.
The problem with natural immunity is the risk of complications. Chicken pox can lead to encephalitis, pneumonia, or, if kids scratch too much, skin infections like MRSA. A polio infection can cause permanent paralysis; mumps, deafness; and Haemophilus influenzae type b (Hib), brain damage.
"Those are the chances people take if they defer the vaccine," Nelson said.
Vaccines aren’t necessary because disease has been eradicated
The only infectious human disease that has been eradicated worldwide is smallpox, according to the World Health Organization (WHO). Even today there are outbreaks of conditions like measles, mumps, and pertussis.
Vaccines can protect you when you’re around those who aren’t vaccinated, either in the U.S or elsewhere. According to the WHO, less than 95 percent of people in many parts of Western Europe receive vaccines, and that’s where 82 percent of measles cases occurred in 2009.
Doctors make money off them
Vaccines aren't a cash cow for docs. "It's probably more of a money loser than anything," said Nelson, because they’re labor intensive. Some doctors do receive financial incentives from HMOs, but "the bonuses are there to support high-quality practice and help the physicians justify the manpower that goes into administering them," she said.
Vaccines are about 1.5 percent of total pharmaceutical revenues, says VaccineEthics.org, a website run by the Penn Center for Bioethics. "We've had problems with vaccine supply because so few pharmaceutical companies are making vaccines anymore," Nelson said. (Three decades ago, more than 30 companies produced vaccines; today about five companies account for 80 percent of the market.)