New Kaiser Permanente study suggests under-vaccination is only one factor contributing to whooping cough outbreaks.
OAKLAND, Calif. — New Kaiser Permanente research published in the journal Pediatrics finds children who were up to date on their pertussis vaccine schedule were far less likely to develop the disease than unvaccinated children. The research also finds that most pertussis cases were in fully vaccinated children and that the risk of illness increased with the time elapsed since vaccination. These findings suggest that waning effectiveness between doses was a significant contributor to recent outbreaks.
The study, “Acellular Pertussis Vaccine Effectiveness Over Time,” was published on June 10, 2019.
Pertussis, widely known as whooping cough, is a highly contagious and potentially life-threatening respiratory infection caused by the bacterium Bordetella pertussis. To help prevent it, the Centers for Disease Control and Prevention recommends 5 doses of DTaP vaccine — a combination vaccine that protects against pertussis, diphtheria, and tetanus — between the ages of 2 months and 6 years
“Most DTaP research has explored either vaccination status or waning effectiveness, but we looked at both at once,” said Ousseny Zerbo, PhD, lead author of the new study and a staff scientist with the Vaccine Study Center at Kaiser Permanente Northern California’s Division of Research.
Dr. Zerbo and his colleagues retrospectively analyzed the electronic health records of 469,982 children under the age of 11 who were members of Kaiser Permanente in Northern California. Focusing on data from January 2006 through June 2017, they performed a series of statistical analyses to determine risk of pertussis according to vaccination status and the time since a child’s last dose.
The researchers found that risk of pertussis was 13 times higher for children who had never received DTaP than for children who were fully vaccinated with all recommended doses for their age. Under-vaccinated children — those who had received at least one dose but were behind schedule — were almost twice as likely to develop pertussis than were fully vaccinated children.
“However, for fully vaccinated children, we found that risk of pertussis increased as more time passed since their last dose,” Dr. Zerbo said. Among age-appropriately vaccinated children between the ages of 19 months and 7 years the risk of pertussis was 5 times higher when they were more than 3 years from their last vaccine dose. Notably, most children diagnosed with pertussis in the study were up to date on DTaP. Of 738 pertussis cases, 603 were fully vaccinated, 99 completely unvaccinated, and 36 were partially vaccinated but behind schedule.
These findings suggest that in a U.S. population where vaccine coverage is high, waning DTaP effectiveness is a significant driver of outbreaks, including the 2010 and 2014 outbreaks in California that each resulted in more than 9,000 pertussis cases.
“The big question has been whether pertussis outbreaks are due to under-vaccination, as in other diseases like measles, or to waning immunity,” said Nicola P. Klein, MD, PhD, senior study author and director of the Vaccine Study Center. “The answer is that both factors matter.”
More research is needed to quantify the relative roles of vaccine waning and under-vaccination in pertussis outbreaks. Still, the new findings point to the need for a better vaccine that lasts longer between doses. Meanwhile, the authors emphasize that families and clinicians should continue vaccinating children according to the CDC’s recommended schedule.
The researchers emphasize that those vaccinated had lower rates of pertussis than those not vaccinated at all.
“Despite increased media attention on parents choosing not to vaccinate, children in our study had high coverage, which was not completely surprising,” Dr. Zerbo said. One percent of the children with pertussis were unvaccinated, and 3 percent were under-vaccinated. “We need to continue to encourage vaccination, no doubt about it,” he said.
This study builds on a growing body of research from the Vaccine Study Center beginning with a 2012 study in the New England Journal of Medicine that found that protection against pertussis after 5 DTaP doses wanes rapidly among school age children who had only ever received DTaP vaccines. This was followed by 2016 and 2017 studies in Pediatrics, one that showed that the routine adolescent Tdap booster also wanes rapidly, and one that showed that Tdap given to pregnant women was highly effective in preventing pertussis in newborns.
Vaccination is an active area of study for Kaiser Permanente Research.
Funded by the National Institute of Allergy and Infectious Diseases, the new study adds to the Vaccine Study Center’s pioneering and influential body of work on pertussis vaccines. Additional authors of the study include Joan Bartlett, MPH, MPP; Kristin Goddard, MPH; Bruce Fireman, MA; and Edwin Lewis, MPH, of the Kaiser Permanente Vaccine Study Center in Oakland, California.
The Kaiser Permanente Division of Research conducts, publishes, and disseminates epidemiologic and health services research to improve the health and medical care of Kaiser Permanente members and society at large. It seeks to understand the determinants of illness and well-being, and to improve the quality and cost-effectiveness of health care. Currently, DOR’s 600-plus staff is working on more than 400 epidemiological and health services research projects. For more information, visit www.dor.kaiser.org or follow us @KPDOR.
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