In a study published in PLOS Computational Biology on April 23rd, a research team led by Manoj Gambhir found that a relatively small difference in efficacy and duration of protection for the first three doses between the acellular pertussis (DTap) vaccine and whole cell (DTP) vaccine (80% for acellular versus 90% for whole cell) to be the most likely explanation for increased incidence of pertussis in the United States over the past ten years.

Photo credit: CDC/ Amanda Mills
Photo credit: CDC/ Amanda Mills

Other explanations such as evolving Bordetella bacteria, declining vaccination rates, decline of natural boosting or increased reporting rates were found to be less likely based on analysis of incidence data from the National Notifiable Disease Surveillance System (NNDSS) from 1950-2009 in this study.

The researchers went on to suggest that waning immunity from the less efficient acellular vaccine was likely most responsible for the recently observed shift in incidence to younger children versus older adolescents.

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The acellular pertussis DTaP vaccine replaced the whole cell DTP in the 1990s to address reactivity issues in patients; after near-elimination of pertussis in the 1970s, incidence has been increasing steadily since the 1990s to recent years where case counts have reached pre-vaccination levels observed in the 1940s. This study concludes that it will likely be necessary to adjust age-spacing of doses for the five-dose childhood schedule and make recommendations for added boosters later in life.

Steven Smith, M.Sc. is an Infectious diseases epidemiologist