Study Finds Tdap Vaccine Loses Effectiveness After 1 Year

Researchers from Northern California Kaiser Permanente examined the pertussis outbreaks that occurred in California between 2010 and 2014 to determine the effectiveness of the tetanus toxoid, reduced diphtheria toxoid, and acellular pertussis (Tdap) vaccine when given to adolescents as a booster. They followed members between the ages of 10 and 19 years, 96.5% of who had received the Tdap vaccine by the age of 14 years.

The analysis found 1207 cases of pertussis (also known as “whooping cough”) among members of the HMO, despite this near-universal rate of vaccination. The study determined vaccine effectiveness (VE) for each year following a booster dose, and found that Tdap VE was 68.8% 1 year out, decreasing to 56.9% during year 2, 25.2% during year 3, and 8.9% during years 4+ from vaccination. The researchers determined that the risk of pertussis increased by 35% each year after the most recent dose of the vaccine was received.

According to the 2016 vaccination recommendations from the CDC/AAP, released earlier this week, children should receive the diphtheria-tetanus-acellular pertussis (DTaP) vaccine at ages 2, 4, 6, and 15 to 18 months, and then again at age 4 to 6 years. These same guidelines advise adolescents receive a Tdap booster between the ages of 11 and 12 years.

The authors theorize that future outbreaks of pertussis will be larger, despite extremely high vaccination rates. Their study age range was designed specifically to test the incidence of pertussis among a population of adolescents who had received only acellular pertussis vaccines (DTaP/Tdap). During the 1990s, they wrote in their study analysis, the United States switched from whole cell pertussis to acellular pertussis vaccines. Since this time, they noted, there have been larger outbreaks of the disease in several countries with extremely high vaccination rates.   

“This study demonstrates that despite high rates of Tdap vaccination, the growing cohort of adolescents who have only received acellular pertussis vaccines continue to be at high risk of contracting pertussis and sustaining epidemics,” they said. “In 2010, children aged 10 to 12 were the first cohort to have exclusively received DTaP vaccines and they were the population at highest risk of disease. The strategy of routinely vaccinating adolescents to prevent future disease did not prevent the 2014 epidemic, arguably because the protection afforded by a dose of Tdap was too short-lived.”

The authors concluded that, rather than mandating the Tdap booster by the seventh grade, as is the case in California, it may be more useful to give when anticipating local outbreaks of pertussis. “The results of this study,” they asserted, “raise serious questions regarding the benefits of routinely administering a single dose of Tdap to every adolescent aged 11 or 12 years.”

The full study was published today online ahead of print, and will appear in the March issue of Pediatrics.

—Drew Amorosi

Reference

Klein NP, Bartlett J, Fireman B, et al. Waning Tdap effectiveness in adolescents. Pediatrics. 2016;137(3):e20153326 [epub ahead of print].