Whooping Cough Booster Declines Rapidly Over 4 Years

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A new, elegantly-designed study sought to document the degree of loss of efficacy of the booster shot against pertussis, or whooping cough, among teens. Unfortunately, rapid decline was in fact found.

The study, led by Dr. Nicola P. Klein and her colleagues based at the Kaiser Permanente Northern California (KPNC) group, was entitled "Waning Tdap Effectiveness in Adolescents," and was published in the journal Pediatrics.

Pertussis is caused by the bacterium Bordetella pertussis. The older version of the vaccine DPT), which was more protective against pertussis used whole-cell antigens to induce strong immunity. Occasionally this caused uncomfortable, but non-life-threatening fevers. Nevertheless, the new acellular vaccine essentially replaced the older one in the U.S. and Europe around 1991. Although it has fewer side effects, the newer vaccine is simply neither as protective nor as long-lasting as the older one.

The DTaP vaccine is for children (usually infants) under seven to inoculate them against diphtheria, tetanus and pertussis. Tdap is the booster shot given at age 11 and then throughout life, after roughly every 10 years, to ensure continued immunity through adolescence and adulthood.

The KPNC researchers' rationale was simple: because the effectiveness of diphtheria-tetanus-acellular pertussis (DTaP) vaccine wanes substantially after the fifth dose at ages 4 to 6 years, a substantial fraction of adolescents are now losing protection against pertussis. Despite high Tdap vaccine coverage among adolescents, California nonetheless experienced large outbreaks in 2010 and 2014. We recently covered a similar outbreak in Florida.

They analyzed pertussis risk in relation to Tdap vaccination status among adolescents beginning on their 10th birthday. On the basis of 1,207 pertussis cases, Tdap VE during the first year after vaccination was 69 percent, decreasing to 9 percent by four years after vaccination. Not surprisingly, adolescents who were vaccinated longer ago were 35 percent more likely to test positive for pertussis than those who were vaccinated more recently.

The authors concluded that routine Tdap vaccination did not prevent pertussis outbreaks, even in this relatively highly-vaccinated population. Among adolescents who had only received TdaP vaccines during childhood, Tdap provided only moderate protection against pertussis during the first year and then that protection declined so rapidly that little protection remained 2-3 years after vaccination, and almost none later than that.

Dr. Klein went so far as to make this dire prediction: We expect future pertussis epidemics to be larger as the cohort that has only received acellular pertussis vaccines ages. The results in this study raise serious questions regarding the benefits of routinely administering a single dose of Tdap to every adolescent aged 11 or 12 years.

A vaccine expert at the University of Minnesota had a different perspective on this situation. As Dr. Mark Schleiss told Forbes.com's Tara Haelle, "this does not mean the vaccine strategy with acellular pertussis vaccines has failed. We have saved lives with pertussis vaccine and changed the disease from a ubiquitous and tragically sometimes fatal disease in infants to an annoying but controllable disease in adolescents and young adults. We can t do anything that would compromise that kind of progress.

He noted that other vaccine strategies, such as influenza, also need improvement, but that doesn t mean tossing what we have. We need improved vaccines with longer duration of protection and this will only come from research, but until then we should continue with our current pertussis vaccine strategy because, although imperfect, it saves lives, he said.

I believe that the CDC and its vaccine specialty group, Advisory Committee on Immunization Practices (ACIP), should seriously encourage a return to the much more effective, older whole-cell pertussis vaccine, DTP (which includes of course vaccines against diphtheria and tetanus as well).