During an American Medical Association media briefing on infectious diseases, Dr. Gellin noted that even a small drop in the current immunization rate may lead to a resurgence of infectious diseases that have been nearly eliminated from the United States.

"There's a lot of information out there about vaccines, but not all of it is equally valid," Dr. Gellin said. "The reasons why so many parents have important misunderstandings about vaccines are complex, but the scientific process itself, and the media, are each partly to blame."

"Even though the vast majority of the scientific data, and virtually all doctors, support the fact that vaccines are safe and effective, research on highly unlikely hypotheses about harmful side effects is often taken out of context," Dr. Gellin said. "The media tend to do stories that make highly speculative hypotheses sound not only plausible, but likely. Currently, hypotheses that vaccines cause autism and diabetes are causing alarm among some parents. The best available scientific evidence does not support either of these hypotheses."

The Internet is an increasingly popular way for vaccine misinformation to spread.

"Patients now come into doctors offices with printouts from the Internet about the supposed harm caused by a certain vaccine, believing the information is fact," Dr. Gellin said. "Often, the scientific data does not support the conclusion, but these sites usually contain powerful anecdotes from parents who believe their children were harmed by a vaccine. That's very believable to another parent."

Dr. Gellin explained that there are very real risks from not receiving vaccinations, risks that every parent should understand before they make a decision not to immunize their child. "If enough people refuse vaccination - and it can take a decline of only a few percentage points in the immunization rate - all children in the community are placed at greater risk," he said. "Despite their incredible performance, vaccines are not 100 percent effective. For example, one out of 100 children fully immunized against measles is still susceptible to the disease. Unvaccinated children are at greater risk themselves, and they place these susceptible children, and children who cannot be given the vaccine for medical reasons, at greater risk."

A July 7, 1999, JAMA article cited by Dr. Gellin found that individuals who declined to be vaccinated for measles for religious or philosophical reasons were 35 times more likely to contract the disease than those who received the vaccine. This risk spills over into the community at large. Dr. Gellin also pointed to the examples of Ireland and the Netherlands, where failure to vaccinate children is linked to increase in disease.

Ireland has had a decrease in the measles vaccination rate over the last few years, Dr. Gellin said. Because of this decrease, and the fact that measles is such a highly contagious disease, a measles epidemic is currently raging in Dublin with more than 700 cases and many children being hospitalized. The outbreak is now also spreading to surrounding counties. The Netherlands also recently had a large measles outbreak with more than 3,000 cases and several deaths in one area where a portion of the community declined to immunize their children.

Complacent attitude towards vaccinations a problem

Dr. Gellin says that because infectious diseases have been so well controlled in the United States through the use of vaccines, there has been a shift in perception about the value of vaccines.

"Most parents today have never seen these diseases, and therefore don't know how serious these diseases can be. They see these as diseases of yesteryear," Dr. Gellin said. In the United States, measles has decreased by 99 percent, from 3 to 4 million cases a year in the 1950s and 1960s to 100 cases last year.

"Unless they have traveled overseas, none of the first-year medical students in my class has ever even seen measles," Dr. Gellin added. "What used to be called 'usual childhood diseases' are not usual any longer. In fact, now they're referred to as 'vaccine-preventable diseases.' "

Immunizations that are recommended for all children include polio, measles, mumps, rubella, Hib, hepatitis B, diphtheria, pertussis (whooping cough), tetanus and varicella (chickenpox). All these diseases still exist and remain a health threat for those who have not been immunized.