In October 2012, Nicola Klein and coworkers at the Kaiser Permanente Vaccine Study Center in Oakland, California, investigated the effectiveness of DTaP vaccine during a pertussis outbreak in California that had occurred in 2010 (Klein NP, Bartlett J, Rowhani-Rahbar A, et al., “Waning Protection after Fifth Dose of Acellular Pertussis Vaccine in Children,” N Engl J Med. 2012 Sep 13;367(11):1012-9.).

Investigators correlated the presence of pertussis illness as determined by pertussis-specific PCR with time following DTaP vaccine. They found that children who received DTaP between 4 and 6 years of age were at progressively greater risk in subsequent years. Specifically, the incidence of pertussis was 4.5 percent among 6-year-olds, 12.2 percent among 8-year-olds and18.5 percent among 10-year-olds. This increased incidence of pertussis between 6 and 10 years of age was followed by a sharp decrease in pertussis following the adolescent booster dose between 11 and 13 years of age.

The authors concluded that increased risk was associated with waning immunity following vaccination, not the natural history of pertussis infection. “The sharp increase in the incidence of pertussis among children 8 to 11 years of age, followed by a sharp decrease at 12 to 15 years of age, is not characteristic of the epidemiology of pertussis in unvaccinated persons or in previous outbreaks,” they wrote.

These data are consistent with previous studies in Canada and Australia showing that a switch from DTP to DTaP was associated with a decrease in protection against disease. Control of future outbreaks will likely require pertussis vaccines that evoke higher levels of protection and longer-lasting immunity.

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