May 26, 2009 -- Children of parents who refuse to have their children vaccinated against whooping cough are 23 times more likely to develop the disease than children who get the shots, according to a new study. The U.S. Centers for Disease Control and Prevention says whooping cough now infects more 5,000 children a year.
Authors of the study, which was conducted at Kaiser Permante Colorado's Institute for Health Research in Denver, say the results show a continued need to "to further understand why parents refuse immunizations and to develop strategies for conveying the risks and benefits of immunizations to parents more effectively."
Whooping cough or pertussis -- the medical name for the disease, causes a respiratory infection with an uncontrolled cough with a characteristic whoop or yelp -- said Dr. Jason Glanz, the lead author of the study.
Parents may shun the vaccines over the fear of rare but serious side effects associated with the use of some vaccines in certain individuals.
Many parents who refuse whooping cough vaccine make the argument that their children don't need to be vaccinated because peers -- friends and classmates -- have been vaccinated so all children in the group would be protected by a phenomenon known as herd immunity.
But Glanz said his study suggests that the herd immunity theory doesn't hold for pertussis.
"One of the misperceptions among parents is that the pertussis vaccine doesn't work and that their children are at low risk for infection, and our study showed that both were not true," Glanz said.
Glanz and his colleagues based that estimate on a case control study that compared children whose parents had refused vaccination to children who received vaccinations. The findings were published in the June issue of Pediatrics.
The researchers determined that 11 percent of whooping cough cases in the overall population were linked to vaccine refusal.
Dr. Gregory Poland, director of Mayo Clinic Vaccine Research Group in Rochester, Minn., said this study's results were consistent with others.
"Parents need to know they place their children at a documented and real risk, as well as others, when they refuse vaccines based on inaccurate information," he said.
He added, "This study shows that there is plenty of pertussis out there and it is highly contagious."
Vaccine Refusal Leaves Door Open to Whooping Cough Infection
The CDC said the incidence of pertussis has increased steadily each year since the 1980s and in 2007 the incidence was 3.6 cases per 100,000 population.
The World Health Organization reported more than 200,000 whooping cough deaths in 2000 world-wide.
Initially the symptoms mimic the common cold -- upper respiratory congestion and a cough. But the cough rapidly progressed to the characteristic whoop or yelp in rapids succession or paroxysms. In infants complications included pneumonia, seizures, brain swelling, and death. Adults are also at risk for pneumonia and rib fractures caused by coughing.
The rate of vaccine refusal in the United States remains low, typically measured by counting non-medical exemptions from required school immunizations. But it has increased over the past two decades.
This could be the result of parents' attention shifting from the dangers of vaccine-preventable illnesses to issues of vaccine safety as the diseases become rarer, Glanz and colleagues said.
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.
Infection Risk Soars 20-Fold in Unvaccinated Children
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."
Doctors Should Confront Misconceptions About Pertussis Vaccine
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.