Pertussis Vaccine Appears to Lose Strength Over Time

A new study finds that the effectiveness of the pertussis vaccine wanes in the 5 years after children receive their last dose, which may help explain recent outbreaks.

The United States is on track to have the highest number of whooping cough cases this year since the late 1950s, despite a high rate of vaccination against the condition. Many have suspected that the waning effectiveness of the diphtheria, tetanus, acellular pertussis (DTap) vaccine over time is to blame for recent outbreaks. Support for this hypothesis can be found in a new study published on September 13, 2012 in the New England Journal of Medicine.

The DTap vaccine is typically administered to children in 5 doses, starting at the age of 2 months and ending between the ages of 4 and 6 years. A booster shot (called Tdap) is recommended at the age of 11 or 12 years. Prior to 1997, a whole-cell version of the pertussis vaccine was available, but it was phased out due to unproven concerns over an association with neurological problems and adverse reactions.

The new case-control study, which was conducted between 2006 and 2011, assessed the risk of pertussis in children aged 4 to 12 years who received health care from Kaiser Permanente Northern California during the 5 years following their fifth dose of the vaccine. A group of 277 pertussis-positive case patients was compared with a control group of 3318 pertussis-negative children and a group of 6086 matched controls.

The researchers found that the odds of contracting pertussis increased by 42% each year after receipt of the fifth DTap dose. Based on these findings, if the initial effectiveness of the vaccine were 95%, it would decrease to 71% after 5 years, and if it were 90%, it would decrease to 42% in 5 years. The researchers also found that, on average, pertussis-positive children received their fifth DTap dose earlier than children in both sets of controls.

In addition, a greater portion of older children were pertussis-positive, with most cases occurring between the ages of 8 and 11 years. The number of cases dropped, however, between the ages of 12 and 15 years. (The researchers note that children aged 12 to 15 years during the study period were old enough to have received the whole-cell vaccination as infants, possibly giving them greater protection against pertussis.)

In light of recent whooping cough outbreaks, researchers suggest that administering the Tdap booster shot to younger children may be advisable. However, they point out that developing a new vaccine would constitute a better, more permanent solution.

“Prevention of future outbreaks will be best achieved by developing new pertussis-containing vaccines that provide long-term immunity,” the researchers write.

In the meantime, Nicola Klein, MD, PhD, co-director of the Kaiser Permanente Vaccine Study Center and lead author of the study, emphasized that children should continue to receive the recommended sequence of 5 DTaP shots.

“The DTaP vaccine is effective and remains an important tool for protection against whooping cough for children and the communities in which they live, and following current CDC recommendations remains important,” she said in a press release.