US News and World Report
17 March 2003

The vaccine conundrum. Should parents be concerned about the number of childhood inoculations?

By Katherine Hobson

When Edward Jenner successfully inoculated a young boy against smallpox in 1796, the British Royal Society refused to publish his findings. The practice of vaccination may not seem as bizarre and scary as it did to 18th-century physicians, but it continues to spark controversy. Most recently, parents have been concerned about the new five-disease combination vaccine and about rumors of a link between childhood vaccines and such conditions as asthma and autism. Meantime, the medical community maintains--most recently in the March issue of Pediatrics--that those links don't exist and that vaccines are safe for the overwhelming majority of kids. U.S. News asked some experts to clarify what's known about childhood inoculations. Here are their answers:

Vaccines have been around for hundreds of years. Why all the fuss now?

Immunization is partly a victim of its own success. It has so drastically reduced the incidence of former killers like diphtheria, measles, and polio that most people--and many doctors--today have never seen a case. Without that perspective, they wonder why all those shots are necessary. "People don't realize that routine immunizations are why we don't have these diseases," says Joel Ward, director of the Center for Vaccine Research at the University of California-Los Angeles. "They perceive a greater risk from the vaccine than from the disease."

In addition, there's been an increase in the diagnosis of autism, asthma, and other still-mysterious disorders. Many of those diseases first appear at around the same age that kids are vaccinated, raising fears of a connection.

What, specifically, do vaccine critics fear that the immunizations do?

People worry about the potency of combined vaccines like the MMR--a single shot that protects against measles, mumps, and rubella--or the new, five- in-one Pediarix, which protects against diphtheria, whooping cough, tetanus, hepatitis B, and polio. Others are concerned about the sheer number of vaccines, says Craig Newschaffer, an epidemiologist at Johns Hopkins University, the fear being "that priming the immune pump in some kids is a bad thing." Still others are focused on possible ill effects from mercury found in thimerosal, a preservative once commonly used in some vaccines.

Does the existing medical evidence justify these worries?

A recent study of more than 500,000 kids in Denmark found no connection between the MMR and autism, supporting a raft of smaller studies suggesting the same thing. A study published last month in Britain found no evidence that the MMR shot overloads the immune system. And a study by the Centers for Disease Control and Prevention has found no association between the MMR and asthma. The Institute of Medicine last year said the current immunization schedule of as many as 20 vaccinations by age 2 does not increase the risk of contracting diabetes or various infections. Indeed, researchers say that because some vaccines, like the one for smallpox, are no longer given routinely, children are actually exposed to fewer antigens in vaccinations now than 30 years ago.

Physicians favor combined vaccines like the MMR and Pediarix because they mean fewer shots and doctor visits for kids, and better protection for the population. (Single doses of MMR components are available in Europe but not in the United States.)

What about thimerosal and mercury?

A small study found recently that infants exposed to thimerosal, a mercury-containing preservative once added to vaccines (and still present in some contact lens solutions and throat sprays) didn't have unsafe blood mercury levels, even by the Environmental Protection Agency's very conservative standards. That's encouraging, but larger studies should be done to make sure a small subset of the population isn't susceptible even to tiny levels of organic mercury, says Newschaffer. It's well known that large enough doses of mercury can lead to neurological problems (story, Page 42). Studies have shown, for example, that fetuses consistently exposed to mercury in fish eaten by their mothers had a higher risk of some developmental disabilities but not of autism. Vaccines produced for kids now contain no thimerosal or only trace amounts, while the vast majority of vaccines containing the preservative are no longer on the shelf. To see if a given lot of vaccine has thimerosal, you can consult
http://www.fda.gov/ cber/vaccine/thimerosal.htm#t1.

So are vaccines totally safe?

It's possible that some vaccines produce very rare, but harmful, allergic responses in some people--that's why the government has a fund to compensate victims of those reactions. "Nothing in life is absolute," says Ward. "Even table salt produces a reaction in some individuals." But vaccines are studied in tens of thousands of people before they're licensed. And the risk of allowing these diseases to gain hold again is far greater than any risk from the vaccines, Ward adds. That doesn't mean that those affected should be written off, but the very small risk of injury must be weighed against the public-health benefits.

If the diseases are so rare now, what's the big deal if some kids aren't vaccinated?

Vaccines don't protect only the one getting the shot. They also protect the public at large. When enough people are immunized, there's little chance that any particularly susceptible person--like a newborn or someone with a weak immune system--will come into contact with an infection. That's called herd immunity, and when the percentage of people vaccinated against a particular disease drops, herd immunity begins to fail. There has been a recent outbreak of measles in Ireland, a country where a major measles epidemic killed three children in 2000 and where only 72 percent of the population has been vaccinated against the disease. These diseases are not gone. Whooping cough is on the rise among teens and adults in the United States, putting unvaccinated infants at risk. Disease rates are far higher outside the United States, and given the ease of global travel, one infected person walking around at a major airport could cause an epidemic. "We are always at war with microbes," says Ward. "Why do we have to relearn this lesson?"