Despite the fact that over 95% of kindergarteners in the United States are properly vaccinated against whooping cough (Bordetella pertussis) with the DTaP vaccine, there are persistent reports that the rates of whooping cough have risen during the past few years. Several outbreaks, along with an outright epidemic in Washington state, seemed to indicate that the vaccine’s effectiveness is waning faster than expected.
A recently published study in Pediatrics evaluates reports of increased rates of pertussis in the six years after receipt of the fifth (of five) DTaP doses. These reports suggest that waning of immunity to pertussis from DTaP is occurring before the recommended booster age of 11 to 12 years. The researchers tracked more than 400,000 Minnesota and Oregon children using immunization records and state health department whooping cough data. All of those children were born between 1998 and 2003 and received the recommended series of five DTaP shots, the final one usually given at 4-6 years old.
The key results are:
- For the six years after the last of the series of five shots, 458 Minnesota children (out of 224,378 immunized children) contracted whooping cough. The rate of new cases rose from 16 per 100,000 children in the first year after their most recent DTaP shot to 138 per 100,000 in year six. A child was twice as likely to get whooping cough during the second year than in the first year. By the sixth year, a child was nine times as likely to contract pertussis (compared to the first year).
- In Oregon, there were 89 cases (out of 179,011), which suggests a rate of 6 cases per 100,000 kids in the first year and 24 per 100,000 in the sixth. The data show that a child in Oregon was 30% more likely to get whooping cough in the first year. By the sixth year, they were 400% more likely to contract the disease.
The authors concluded that “this rise (in whooping cough cases) is likely attributable in part to waning immunity from DTaP vaccines.” The waning immunity probably contribute to a larger pool of children who are susceptible to the disease, even though they are vaccinated, which leads to outbreaks in age groups in which whooping cough was previously unknown (or rare).
Part of this increase in pertussis may be because of the use of the acellular version of the pertussis vaccine (the “aP” part of the DTaP vaccine) versus the previous “whole cell” version, which may have provided more protection against pertussis though they had more injection-site and other safety issues.
The authors speculated that the resurgence may result from a natural and cyclical increase in the rate of the disease, increased recognition by physicians, and improved laboratory tests for the disease, along with waning immunity. The authors also state that, “although multiple factors can explain the overall increase in pertussis, the striking and sudden increase in disease among 7- to 10- year-olds beginning in 2005 and the strong cohort effect that is observed in national surveillance data are likely being driven by earlier waning.”
They also dismiss the claim that changes in the B. pertussis bacteria may have contributed (and continue to contribute) to the resurgence in whooping cough. A recent study that evaluated historical B. pertussis strains showed that the organism’s antigen had drifted away from vaccine targets many years before the transition to the acellular pertussis vaccines.
Even though there does appear to be good evidence that there is a waning in the effectiveness of the pertussis portion of the DTaP vaccine (there is no evidence that there is any issue with the diphtheria or tetanus parts of the vaccine), this does not mean that the vaccine is “ineffective” or the cause of the recent outbreaks.
The DTaP vaccine is still extremely effective at prevent pertussis in the short-term against all circulating strains of whooping cough. Furthermore, children who receive the full series of vaccines have milder cases of whooping cough (if they contract it). Most importantly, because babies are most susceptible to complications from whooping cough (for example, about 1.6% of babies who contract the disease die of it), the importance of vaccinating everyone (including adults) who are in contact with babies cannot be ignored.
This is how science works. An issue with waning immunity has been uncovered, and because of it, research is well underway to discover more effective vaccines for pertussis. There may also be a change in the timing of doses for pertussis to improve the effectiveness of the vaccine in the short-term, before a new vaccine (or vaccines) enter the market. Remember, Vaccines Save Lives.
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- Centers for Disease Control and Prevention (CDC). Vaccination coverage among children in kindergarten–United States, 2011-12 school year. MMWR Morb Mortal Wkly Rep. 2012 Aug 24;61(33):647-52. Erratum in: MMWR Morb Mortal Wkly Rep. 2012 Dec 7;61(48):994. PubMed PMID: 22914226.
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- Tartof SY, Lewis M, Kenyon C, White K, Osborn A, Liko J, Zell E, Martin S, Messonnier NE, Clark TA, Skoff TH. Waning Immunity to Pertussis Following 5 Doses of DTaP. Pediatrics 2013 April;131(4):. PubMed PMID: ; PubMed Central PMCID: .
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