The study was led by a vaccine research team at Kaiser Permanente Colorado's Institute for Health Research in Denver and was published online in Pediatrics on 26 May.
The researchers wrote that most parents have their children vaccinated, and this has led to a dramatic fall in numbers of children with serious childhood diseases, but despite this, the number of parents who don't want their children to be vaccinated appears to be rising in the US.
The scientists wanted to find out if children infected with pertussis or whooping cough were more likely to have parents who had refused to let them be vaccinated than equivalent children who did not become infected.
For the case controlled study the researchers used data on children enrolled in the Kaiser Permanente of Colorado health plan between 1996 and 2007. They matched each case of whooping cough to 4 randomly selected controls (ie children who had not had the disease).
The results showed:
- A total of 156 lab confirmed cases of whooping cough and 595 matched controls.
- 18 of the infected cases (12 per cent) were children whose parents had refused to let them be immunized.
- This compared with 3 cases (0.5 per cent) vaccine refusers among the controls.
- Children of parents who had refused the vaccination were at higher risk of infection than children who had been vaccinated.
- A similar pattern between parental vaccine refusal and higher risk of whooping cough was found in a secondary case control analysis of children aged 2 to 20 months who had been continuously enrolled in the same health plan.
- 11 per cent of all whooping cough cases in the whole pediatric population of Kaiser Permanente of Colorado were attributed to parents refusing to let their children be vaccinated.
"Children of parents who refuse pertussis [whooping cough] immunizations are at high risk for pertussis infection relative to vaccinated children."
They suggested that "herd immunity" does not appear to give unvaccinated children complete protection from whooping cough and more research is needed to understand why parents won't allow their children to be vaccinated and to develop effective ways of informing them about the risks and benefits of vaccination.
Whooping cough is a highly contagious bacterial disease that causes violent and uncontrollable coughing. It can be very serious and even fatal, especially in infants under 2 months old who are not old enough to be fully immunized.
In 1976 in the US there were just over 1,000 reported cases of whooping cough, and by 2004 this went up to nearly 26,000 cases, with 140 deaths recorded between 2000 and 2005.
Lead author Dr Jason Glanz, a senior scientist at Kaiser Permanente's Institute for Health Research said:
"This study helps dispel one of the commonly held beliefs among vaccine-refusing parents: that their children are not at risk for vaccine preventable diseases."
"It also shows that the decision to refuse immunizations could have important ramifications for the health of the entire community. Based on our analysis, we found that one in 10 additional whooping cough infections could have been prevented by immunization," he added.
"Parental Refusal of Pertussis Vaccination Is Associated With an Increased Risk of Pertussis Infection in Children."
Jason M. Glanz, David L. McClure, David J. Magid, Matthew F. Daley, Eric K. France, Daniel A. Salmon, and Simon J. Hambidge.
Pediatrics, Jun 2009; 123: 1446 - 1451.
Published online 26 May 2009.
Additional sources: Kaiser Permanente.