Back in July, with whooping cough cases up 1,300 percent in Washington State over last year, we reported that “nationally, vaccinated 13- and 14-year-olds are contracting whooping cough in greater numbers, indicating that the childhood DTaP vaccine‘s protection is wearing off sooner than expected.” (Washington is now at 4,007 cases, compared to last year’s 405 total.)
Now a study from the Kaiser Permanente Vaccine Study Center in Oakland, California, confirms that suspicion. Published in the New England Journal of Medicine, the study, titled “Waning Protection after Fifth Dose of Acellular Pertussis Vaccine in Children,” has measured with a new level of precision the whooping cough (pertussis) vaccine’s effects.
“After the fifth dose of DTaP, the odds of acquiring pertussis increased by an average of 42% per year,” the study’s authors write. Said another way, the overall protection rate of the DTaP vaccine declines to 71 percent from 95 percent by the fifth year after the fifth shot. (A further caveat, though, is that while immunity falls, vaccinated kids don’t get as sick from whooping cough.)
For context, unvaccinated children have odds of getting whooping cough “at least eight times higher than children who received all five doses” of DTaP (CDC pdf).
Previously, this kind of study was difficult to do because of the switch from the whole-cell vaccine to DTaP, which muddied results, and the necessity of making sure everyone had gotten the same number of shots. (Nationally, 95 percent of children get three and 85 percent get four. Fewer get all five.)
The whole-cell vaccine contains inactive B. pertussis cells; while lab tests differ, it seems more effective than DTaP in actual practice. It’s still used around the world (worldwide, it’s estimated that nearly 300,000 children die of whooping cough annually), but the U.S. switched to the bits-of-cells DTaP in 1991 because of its lower rate of–and milder–side effects, including soreness at the injection site and fever.
MedLine Plus breaks out the rates for more severe reactions:
- Non-stop crying for more than 3 hours (1 in 1,000 children)
- Fever over 105 degrees (1 in 16,000 children)
- Seizures (1 in 14,000 children)
(A 2001 study was able to show that mice developed seizures similar to those seen in children who got the whole-cell vaccine.)
The Kaiser Permanente study was able to isolate children who got their five DTaP shots between 47 to 84 months of age, and contrast those who got whooping cough (277) with DTaP-ers who didn’t (3,318), as well as with a larger sample of “matched controls” (6,086) drawn from the general population of Kaiser patients. From this, they were able to determine that the key factor was time; the children who contracted whooping cough were more likely to have gotten their fifth shot earlier, giving their immunity more time to wane.
The goal has long been a more effective vaccine, that would require fewer than five doses. This news only adds fuel to that effort. Adults would likely appreciate an advance in vaccine strength, too. The Tdap vaccine for adults provides only 55 percent protection. Whooping cough is generally less severe for adults, but it can lead to pneumonia if you don’t know you have it.
