In a herd effect, some parents believe the diseases are not much of a threat to their children, or that vaccinated children will protect the unvaccinated, they say.
But Dr. Ari Brown, a pediatrician in Austin, Tex., and co-author of Baby 411, said this study "is proof that their kids cannot hide in the herd."
Accompanying the rise in vaccine refusal are increasing rates of some vaccine-preventable illnesses, such as pertussis and measles. In 2007, there were more than 10,000 cases of pertussis in the U.S. In 2008, there were 131 cases of measles, the most in 12 years.
And, because like-minded families "live in the same communities and send their children to the same schools," Brown said, "it leaves clusters or hot pockets that are ground zero for an outbreak."
For example, Colorado had a disproportionate number of the pertussis cases in 2007, making it a good place to study vaccine refusal and rates of infection, Glanz said.
To quantify the increased risk of pertussis with vaccine refusal using patient data, the researchers examined Kaiser Permanente's electronic health records for patients ages 2 months to 18 years over a 12-year period.
They looked for children who had laboratory-confirmed cases of pertussis, and for those whose parents had refused at least one of the initial series of diphtheria, pertussis, and tetanus (DTaP) immunization for their children.
During the study, there were 156 confirmed cases of pertussis infection. These cases were matched with up to four healthy controls each.
During the study period, the overall rate of vaccine refusal at Kaiser Permanente was less than 1 percent.
Infected children were significantly more likely to have parents who refused vaccinations than the controls (11.5 percent versus 0.5 percent). The difference equated to a 22.8-fold increased risk of infection in the unvaccinated children.
In a secondary analysis limited to children who were continuously enrolled at Kaiser Permanente from 2 to 20 months of age (the period during which the initial immunization series would be completed), there was a similarly high risk.
Glanz said that when clinicians encounter parents who refuse vaccines for their children, "it's important to not be defensive, not be condescending. Just acknowledge their fears and help them weigh the risks and benefits the best way you can."
Glanz also said it might be helpful for physicians to familiarize themselves with the vaccine-refusal literature so they can directly refute misconceptions -- although he acknowledged that time constraints make this difficult.
The authors acknowledged that the study was limited by the use of data from a single health plan in one state.
The study was supported by a grant from the National Institute of Allergy and Infectious Diseases.
The authors reported that they had no relevant financial relationships to disclose.