Vaccinated but still getting pertussis

Rapidly waning protection among teenagers who have received the tetanus, diphtheria, acellular pertussis vaccine (Tdap) is apparently contributing to an upsurge in pertussis (whooping cough) in the United States, according to 2 recent studies. The incidence of pertussis has increased steadily over the past decade; 2012 saw the highest number of cases since 1955-48,277.

A large portion of the 2012 cases-almost 5000-occurred in the state of Washington during the worst pertussis epidemic there since 1942. The infection rate was surprisingly high among older children (aged 7 to 10 years) and adolescents.

A matched case-control study conducted during the epidemic examined the effectiveness and duration of protection of Tdap among Washington residents born between 1993 and 2000 who had received 5 doses of whole-cell or acellular vaccine on schedule. (Acellular vaccine, DTaP, replaced whole-cell pertussis vaccine for all 5 childhood vaccine doses after 1997. The Tdap vaccine was added to the vaccine schedule for adolescents and adults in 2005.)

Based on birth year, participants were divided into 2 groups according to the type of vaccine received: a mix of whole-cell and acellular (1993 through 1997) or all acellular (1998 through 2000). Overall Tdap effectiveness among adolescents who received all acellular vaccine (450 cases, 1246 controls) was 63.9%. Effectiveness dropped markedly over time from 73% at 12 months after vaccination to 54.9% at 12 to 23 months to 34% at 24 to 47 months.

The sharp decline in efficacy over 2 to 4 years led the researchers to conclude that “lack of long-term protection after vaccination is likely contributing to increases in pertussis among adolescents.”

A second study that applied mathematical modeling to 60 years of pertussis data reached a similar conclusion with regard to the national resurgence in pertussis cases. The analysis found that Tdap offers a lower level of protection than the whole-cell vaccine: 80% for the first 3 doses of acellular vaccine compared with 90% for the whole-cell vaccine.

The researchers conclude that the difference in efficacy and protection duration between the vaccine types is the model that best explains the incidence data and predicts pertussis incidence between 2010 and 2012. Other hypotheses, such as increased reporting of pertussis cases over time, don’t provide a sufficient explanation of the surge in incidence, they say.

The researchers note that the difference in efficacy between Tdap and whole-cell vaccine isn’t large, suggesting that “booster doses may be sufficient to curtail epidemics while vaccine research continues.”