The Advisory Committee on Immunization Practices will soon be considering an alternate revaccination schedule for the tetanus-diphtheria-acelluar pertussis vaccine to improve the vaccine’s waning coverage of pertussis.
“There has been a tremendous reduction in the occurrence of pertussis in the United States since the vaccine’s inception,” Thomas Clark, MD, medical epidemiologist for the CDC, said during a presentation. “The burden has been reduced substantially, but in recent years we’ve seen increases in pertussis with notable epidemic years.”
According to Clark, preliminary data show that the national overall pertussis incidence in 2012 was 13.4 cases per 100,000, with the rate reaching as high as 100 cases per 100,000 in some states. This is despite a high coverage with childhood vaccinations, with 96% of children aged 19 months to 35 months having had three or more doses of the vaccine.
Infants continue to have the highest risk for pertussis and are most likely to be hospitalized with the disease. In the 1990s, there was an emergence of disease among adolescents, and then in 2000, an emergence among fully-vaccinated children aged 7-10 years. In 2012, there was again more adolescent disease.
“Rates of disease in infants are higher now than they were before the Tdap recommendation,” Clark said. “We don’t feel like we have a herd effect of Tdap coverage in adolescents.”
In the 1990s, whole cell preparations of pertussis vaccines were phased out and replaced with acellular vaccine. This was originally only for the booster doses, but the entire series changed to acelluar preparations in 1997.
The resurgence of disease in the 2000s among vaccinated school-aged children is likely due to waning of protection from the acellular vaccine, Clark said. The disease is reemerging in adolescents despite high coverage with Tdap, suggesting that the Tdap boosting may wane more quickly than anticipated.
“We feel that the switch to acelluar pertussis vaccines may be changing the epidemiology of pertussis,” Clark said. “What we’re seeing is a problem of susceptibility despite vaccination, not a problem of vaccination.”
In reviews of antibody persistence with Tdap vaccination, for both diphtheria and tetanus, there were rapid rises in antibody protection immediately upon vaccination, with small declines over the first year and a plateau through to 10 years. Pertussis antigens, however, saw a substantial decline in the first 2 years since vaccination and a consistent slower decline through to 10 years.
“We are reassured that protection against diphtheria and tetanus persist at 5 and 10 years, but are concerned about the rapid decline in pertussis antibodies in the first 3 years,” Clark said.
The working group is now considering data to determine the safety and cost-effectiveness of shorting the intervals between revaccination with the Tdap vaccine to combat waning immunity to pertussis. They hope to have recommendations for the ACIP meeting in June.