Children become more vulnerable to infection with pertussis the more time passes after their last dose of the acellular pertussis vaccine, according to a study published in JAMA today.
After a record-breaking epidemic of pertussis in California in 2010, many public health officials became curious why so many 7- to 10-year-olds became ill, even though most had been vaccinated with the diphtheria, tetanus, and acellular pertussis vaccine (DTaP). The acellular vaccine was added to the recommended vaccination schedule in the United States in 1997, replacing the whole-cell pertussis vaccine, because the acellular version was associated with fewer adverse effects. But some recent evidence suggests that the newer, safer vaccine may not protect as well as the older version.
To probe whether protection from the 5-injection DTaP series may wane over time, researchers from the US Centers for Disease Control and Prevention and the California Department of Public Health analyzed data on 682 pertussis cases among children aged 4 to 10 years and a control group of 2016 children without pertussis. The researchers found that 7.8% of the cases and 0.9% of the controls had not received any of doses in the DTaP series. Cases were less likely than controls to have received their final dose of pertussis vaccine in the past year; only 2.8% of cases had received a dose within this time period compared with 17.6% of controls. The more time that had elapsed since a child had received a DTaP dose, the likelier he or she had contracted pertussis. For example, 33.9% of cases and 14.3% of controls had received their last vaccination at least 5 years prior to the study.
Lara K. Misegades, PhD, MS, of the CDC’s meningitis and vaccine preventable diseases branch, discussed the findings with news@JAMA.
news@JAMA: What does your study on the 2010 pertussis outbreak tell us about the protection offered to children by the DTaP, the acellular vaccine?
Dr Misegades: Overall, vaccine protection was 89%. The vaccine was 98% effective in kids within a year of receiving their fifth dose. But when we looked at each year after that, we saw a decrease in vaccine effectiveness. By the time we got to 5 or more years after vaccination, efficacy had dropped to 71%.
news@JAMA: Initially, some hypothesized that California’s pertussis epidemic was being driven by unvaccinated individuals. Do your data support this?
Dr Misegades: Our data don’t support that. Unvaccinated individuals made up approximately 8% of cases and about 1% of controls. We didn’t see that unvaccinated children were driving the epidemic, but they did have a higher risk of pertussis infection than those who were vaccinated.
news@JAMA: Was the California epidemic extraordinary, or was it part of a larger trend?
Dr Misegades: We did see a high incidence in other states that year, but because California has such a large population, they had a very high number of cases. We continue to see other states with high levels of pertussis. In the United States this year, there have been 36 000 cases reported to date, the most cases ever. Pertussis tends to cycle with outbreaks every 2 to 5 years and not always in the same geographic areas.
news@JAMA: This study seems to be the latest to suggest that the whole-cell vaccine may have been more effective than the new acellular version, despite having more adverse effects. What do you think the evidence as a whole is telling us about the differences between these 2 vaccines?
Dr Misegades: We don’t really have comparable studies of coverage over time for the whole-cell vaccines, but there is some evidence they may have provided more coverage over time. We hope to gain a better understanding of this.
news@JAMA: Do you think your findings suggest that another pertussis booster may be needed for older children?
Dr Misegades: In addition to the study released today, we have some ongoing studies of the tetanus, diphtheria, and pertussis (Tdap) booster, which is given to children between ages 11 and 12. It would be helpful to get the results of those studies.
There is not a lot of room to move the doses around in the schedule. The fifth DTaP dose is given at school entry, so we risk reducing uptake if we move it. Tdap also was only licensed for individuals aged 10 years and older. We are still evaluating all the evidence and considering our best strategies.
news@JAMA: What is the main message you think physicians and parents should take away from the study?
Dr Misegades: The study provided the first estimation of vaccine effectiveness over time for the 5-dose DTaP series. While this vaccine is working well, its effectiveness declines over time. The vaccine is our best tool to prevent pertussis right now. The data also demonstrate the importance of completing the series and making sure children get the booster at age 11 or 12.