Changes to pertussis immunization may weaken immunity against whooping cough.
In research published in the journal Pediatrics, scientists confirmed that rates of disease increased after the final of five doses of the pertussis vaccine, known as DTaP, climbing successively in each of the following six years.
“You get a little less protected with each additional year, the farther out you are from the vaccination,” says Sara Tartof, now a vaccine researcher at Kaiser Permanente Southern California who was at the Centers for Disease Control and Prevention when she conducted the study.
Tartof studied children in Minnesota and Oregon who received all five DTaP doses and compared their health records to data collected from the region on pertussis cases. She and her colleagues found that pertussis increased in children in the six years after they received their final dose, which strongly suggested that immunity to the disease provided by the immunization was weakening.
Children typically receive their final DTaP dose between the ages of 4 and 6, so Tartof zeroed in on pertussis cases occurring in children between the ages of 7 and 10. In Minnesota, she tracked 224,378 children. In the first year after complete immunization, there were 15.6 cases of pertussis per 100,000 children; by the sixth year, the rate was 138.4 cases per 100,000 children.
In Oregon, the incidence of pertussis also increased among the 179,011 children followed, although at a slower rate than in Minnesota. That could be due to different surveillance methods or the way pertussis manifests; cases of the disease typically rise and fall in various states. The data mirrored previous data from an outbreak of pertussis in California, which also showed that protection waned after the final dose of the shot.
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What was causing the drop in protection? Part of the problem may have to do with the way the vaccine is formulated. About 15 years ago, vaccinologists switched to the acellular vaccine, which includes only a few of the many potential targets on the surface of the pertussis bacterium that could potentially activate the immune system. The previous shot, which presented the entire killed bacterium to the immune system, produced more long-lasting immunity but also triggered more side effects such as limb-swelling. Experts suspect that fewer pertussis targets may lead to a weaker immune response that doesn’t persist as long.
In fact, the researchers noticed a rise in pertussis cases among the 7- to 10-year-olds in the study; these children were among the first to be exposed to the new acellular vaccine, in the 1990s, starting from their first dose at two months old.
To address the waning immunity, as outbreaks of pertussis erupted in Wisconsin, Vermont and Minnesota in recent years, CDC officials changed their immunization recommendations in 2012 to include a booster dose with the older shot for those older than 19 years, as well as a sixth dose for children, at age 11 or 12.
The results don’t mean that the DTaP vaccine isn’t worth getting. While the protection isn’t ideal, Tartof says that with no new pertussis vaccines in the pipeline, parents should continue to vaccinate their children with the available acellular doses. “It’s the best we have,” she says.
What’s more, vaccinated kids who do wind up getting pertussis tend to have milder symptoms than those who aren’t vaccinated. Experts say that the ability to stave off more severe disease is reason enough to get kids immunized. And adults need protection too: all pregnant women should be vaccinated, along with any adults who come into contact with babies since newborns can’t receive their first dose until two months of age.