The odds of a child developing pertussis increase as the interval since he or she received the fifth and final dose of the DTaP vaccine increases, according to a report published in the November 28, 2012 issue of JAMA.
This pattern indicates a progressive waning of vaccine effectiveness every year after completion of the vaccine series, which would explain the recently noted surge in pertussis cases among 7- to 10-year-olds in at least 34 states, said Lara K. Misegades, Ph.D., of the Meningitis and Vaccine Preventable Disease Branch of the Centers for Disease Control and Prevention and her associates.
Lara K. Misegades, Ph.D.
These are the findings of "the first large-scale assessment of the U.S. five-dose DTaP schedule conducted in the setting of a mature vaccination program and allowing for a comparison of fully vaccinated and unvaccinated children." Together with the findings of previous studies that used different methods to examine this issue, the results "suggest that waning of immunity following DTaP vaccination may have resulted in a much larger pool of susceptible individuals" than previously realized.
"In periods of increased pertussis transmission, the burden of disease attributable to the vaccinated but susceptible population is high," Dr. Misegades and her colleagues noted.
The investigators examined the durability of the protection provided by the DTaP vaccine by using a case-control study design to compare pertussis incidence between fully vaccinated and unvaccinated children aged 4-10 years who were living in California during the 2010 pertussis epidemic there. The study population comprised 682 cases who developed confirmed, probable, or suspected pertussis and 2,016 controls who did not, enrolled from the offices of 265 clinicians.
All the vaccinated children had received their first three doses before the age of 1 year, a fourth dose at 1-2 years of age, and a fifth dose at 4-6 years of age.
Compared with controls, children who developed pertussis had a lower chance of having received all five doses of DTaP, with an odds ratio of 0.11. This finding was not unexpected.
When the study subjects were categorized by time since completion of the vaccine series, using unvaccinated subjects as the reference group, those who developed pertussis were less likely to have received their fifth dose within the preceding 12 months. The rate of pertussis was 2.8% among children who had received their final dose of vaccine during the previous year, compared with 17.6% among children who had received their final dose of vaccine more than 1 year previously, the investigators said (JAMA 2012;308:2126-32).
This association not only persisted but became stronger with increasing time since receipt of the final vaccine dose, so that vaccine effectiveness declined further with each succeeding year.
To adjust for a possible misclassification bias, which is inherent in all case-control studies, a secondary analysis was performed using only cases of confirmed pertussis, excluding cases of probable or suspected pertussis. The results "did not change appreciably," indicating that the data were not affected by misclassification bias.
These findings, together with those of previous studies using different methods of analysis, "have raised concerns about the current U.S. pertussis vaccine program and may prompt consideration of alternative schedules. Options include delaying administration of the fifth DTaP dose or administering the Tdap booster at earlier than 11 years of age," Dr. Misegades and her associates said.
"These issues will require careful and ongoing review of the epidemiology and vaccine program nationwide. Ultimately, improved control of pertussis may require a vaccine that provides longer duration of protection or differently affects transmission in the community," they added.
No financial conflicts of interest were reported.