Protection against the disease pertussis, or whooping cough, doesn’t appear to be as strong with the currently administered vaccine when compared with the older version administered up until the 1990s, according to a new study in Pediatrics. During a pertussis outbreak in 2010–11 in California teens who had received four doses of the current vaccine were at almost six times more likely to get pertussis as those who had received four doses of the older preparation.
The study offers more precise evidence that the earlier pertussis vaccine was superior to the current one, building on previous research that had already shown that the newer vaccine’s effectiveness wanes sooner than expected. The new formula’s reduced effectiveness is most likely driving the recent epidemics of pertussis, says the study’s lead author, Nicola Klein, co-director of Kaiser Permanente Vaccine Study Center in Oakland, Calif. After more than three decades that never saw annual U.S. pertussis cases top 10,000 (they usually totaled fewer than 3,000), the number of cases began climbing rapidly in the late 1990s. The 27,550 cases in 2010 were the highest since 1959, and the preliminary 2012 numbers, at 41,880, were the highest since 1955.
The lower level of protection offered by the current vaccine, however, does not mean it isn’t effective, says Paul Offit, director of the Vaccine Education Center at the Children’s Hospital of Philadelphia. “You’re still eightfold less likely to get pertussis if you’ve gotten the acellular vaccine than if you didn’t,” he says. “It’s an imperfect vaccine, but it’s the best that we have.”
Out with the old
Pertussis is a bacterial infection of the lungs that leads to uncontrollable, deep coughs. The condition can be fatal for very young children. The current vaccine is acellular, meaning it contains protein pieces of the Bordetella pertussis bacterium, given in combination with the tetanus and diphtheria vaccines in the DTaP formulation for infants and children up to six years old. The adolescent and adult booster is a similar compound called Tdap. The older vaccine, DTP, used whole cells of the pertussis bacterium but had more dramatic side effects, often causing high fevers and sometimes fever-induced seizures.
Although no evidence has shown that the seizures caused any long-term damage, they were frightening and led scientists to develop the acellular version. Phased in throughout the U.S. during the 1990s, the acellular vaccine was officially adopted in the U.S. Centers for Disease Control’s childhood immunization schedule in 1997. At Kaiser Permanente Northern California, which conducted the study, the first use of the new DTaP was in the vaccine’s fifth booster shot in 1991, followed by its use for the fourth booster the next year. The three-dose initial series of DTaP began in 1997; by 1999, all five shots for kids under age 10 used the DTaP.
This study focused on the children who got their five-shot series during that transitional period between vaccines. The researchers examined the medical records of all children born from 1994 to 1999 who received all five shots—the first four before age two and the fifth by age seven—and who were in the Kaiser Permanente system during the pertussis outbreak from January 2010 to December 2011.