Targeted Tdap reduced pertussis incidence, increased coverage

Skoff TH. Arch Pediatr Adolesc Med. 2012;doi:10.1001/archpediatrics.2011.1093.

Issue: February 2012

Pertussis incidence decreased among those aged between 11 and 18 years who received targeted tetanus-diphtheria-acellular pertussis vaccination, but increased coverage of adolescent and adult populations is necessary to reap the full direct and indirect benefits of the vaccine, according to CDC data published online.

Tami H. Skoff, MS, and colleagues from the CDC examined data from the CDC’s Nationally Notifiable Diseases Surveillance System. They said 200,401 cases of pertussis were reported in the United States from 1990 to 2009, of which 66.5% were classified as confirmed and 26.7% were classified as probable.

Skoff and colleagues said there was “a steady increase in pertussis incidence among adolescents 11 to 18 years old compared with all other age groups before Tdap introduction.” Incidence increased from 1.4/100,000 to 12.3/100,000 from 1990 to 2003, then increased to 26.5/100,000 in 2004.

Consequently, based on the data, it was discerned that adolescents also had “a steep decreasing trend” once Tdap was introduced, which highlighted the vaccine’s particular effectiveness in this population; the rate decreased 70.7% in this population from 2005 to 2007.

Reduction in infant pertussis cases were not observed during the study period, but the researchers said they were not surprised because adolescent and adult Tdap coverage needs improvement.

For the full benefit of Tdap vaccine to be realized, thorough coverage of both adults and adolescents is needed, the researchers concluded.

Disclosure: The researchers report no relevant financial disclosures.

Roger Baxter, MD

As pertussis incidence in the US continues to rise, researchers are wondering why the vaccines haven’t slowed it down. In 2005, vaccines with tetanus, reduced diphtheria, and acellular pertussis antigens (Tdap) were licensed for use in adolescents and adults. It was hoped that immunizing this group could help combat waning immunity following prior vaccination, and possibly prevent spread of the infection to infants, the age group most susceptible to serious infection with the pertussis bacterium. This study by Skoff and colleagues describes patterns of pertussis incidence in adolescents, compared with other age groups, from1990 to 2009, periods of time pre-dating, and after the introduction of Tdap.

The study showed that the incidence of pertussis increased over the years preceding Tdap, and then declined for a time after its introduction. This is reassuring and hopefully reflects the effectiveness of Tdap at preventing disease in this age group.

Unfortunately, it is difficult to tell from the study just how well the vaccine actually works. The main reason for this is that both the age groups studied and the disease itself are “moving targets.” In the 1990’s, acellular pertussis vaccines gradually replaced the older whole cell versions. Over time, we have only now, in the past 2 years or so, arrived at a cohort of children entering adolescence who have received exclusively acellular vaccines. These vaccines have fewer side effects compared with the early whole-cell vaccines, but we now think they are not as effective, and may wane more rapidly over time. Even so, if we are moving into an era of a less-effective vaccine, it is good to see that the researchers found a decrease in disease activity at one point after its introduction.

The second “moving target,” however, is pertussis, the disease itself. We know that pertussis is cyclic, with patterns of outbreaks every few years. Therefore, looking just a few years after large scale use of Tdap (which really began around 2006) may not give us a clear picture of what is to come. And in fact, in 2010, we saw record numbers of cases, particularly in California.

Lastly, Tdap was licensed and recommended, at least in part, to help prevent infections in infants, who are most likely to suffer serious consequences. The researchers found no benefit for this group. Perhaps that will be seen more clearly when new data from more recent outbreaks are analyzed. The actual benefit of acellular booster vaccines may be easier to determine once we study a group of children who have never received any whole-cell vaccine. Time will tell.

– Roger Baxter, MD

Co-Director of the Vaccine Study Center at Kaiser Permanente

Oakland, Calif.

Disclosure: Dr. Baxter reports no relevant financial disclosures.

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