Children who were up to date on their vaccine schedule were less likely to develop the disease than unvaccinated children, but in most pertussis/whooping cough cases the children were fully vaccinated; risk of vaccinated children becoming ill increased with time since vaccination, suggestion waning effectiveness is a significant contributor to recent outbreaks.
“Most DTaP research has explored either vaccination status or waning effectiveness, but we looked at both at once,” says Ousseny Zerbo, PhD.
Electronic health records of 469,982 children under the age of 11 who were members of the Kaiser Permanente Northern California were retrospectively analyzed, performing a series of statistical analyses to determine risk of whooping cough according to vaccination status and time since the last dose.
Risk of pertussis was found to be 13 times higher for those who didn’t receive DTaP than those who were vaccinated; those who were behind one dose were twice as likely to develop pertussis than fully vaccinated children; and for fully vaccinated children risk increased as more time passed since their last dose.
Fully vaccinated age appropriate children between the ages of 19 months and 7 years old were 5 times more likely to develop pertussis when they were more than 3 years from their last dose; of 738 pertussis cases 603 were fully vaccinated, 99 completely unvaccinated, and 36 were vaccinated but behind on their schedule.
Based on these findings where vaccine coverage is high waning DTaP effectiveness is a significant driver of outbreaks, including the 2010 and 2014 outbreaks that each resulted in more than 9,000 pertussis cases. This study adds to a growing body of research finding protection against pertussis from vaccination wanes rapidly among school aged children.
“The big question has been whether pertussis outbreaks are due to under-vaccination, as in other diseases like measles, or to waning immunity.The answer is that both factors matter.” says Nicola P. Klein, MD, PhD, director of the Vaccine Study Center.
“Despite increased media attention on parents choosing not to vaccinate, children in our study had high coverage, which was not completely surprising,” says Dr. Zerbo.
