Sandra Yin

May 19, 2011

May 19, 2011 (Baltimore, Maryland) — The protective power of the diphtheria, tetanus, acellular pertussis vaccination (DTaP) wanes with time, according to a study presented here at the National Foundation for Infectious Diseases 14th Annual Conference on Vaccine Research.

"The most surprising finding is that the vaccine wanes as much as it does over time," lead author Roger Baxter, MD, told Medscape Medical News. He is codirector of Kaiser Permanente's Vaccine Study Center in Oakland, California, where he helps track and monitor the safety of vaccines for the US Centers for Disease Control and Prevention in Atlanta, Georgia.

Each year that elapses after vaccination is associated with a 36% increased risk of acquiring pertussis, he said.

"If you look over time, this means that whatever your DTaP vaccine was worth to begin with, at 3 years it's 32% and at 5 years it's 16% of your initial effectiveness," Dr. Baxter told meeting attendees.

The study, which is based on a population of 22,700 Kaiser Permanente of Northern California members, 4 to 11 years of age, was conducted between December 2005 and December 2010. Researchers analyzed pertussis rates, confirmed with polymerase chain reaction (PCR), and assessed the risk for a positive pertussis test in relation to days elapsed since the fifth dose of DTaP. In 2010, California saw its highest rate of pertussis in more than 50 years.

PCR tests revealed that 5.7% of tests were positive for Bordetella pertussis. Rates were highest in infants and in children 8 to 12 years of age. Decreasing rates in children 10 to 14 years coincided both with the use of the adolescent formulation of Tdap (tetanus, diphtheria, pertussis vaccination), rather than DTaP, and with the use during infancy of whole-cell vaccines.

In a related finding, the researchers found that acellular pertussis vaccines appeared to offer much less protection than whole-cell vaccines.

Dr. Baxter explained that the mix of proteins in the vaccine plays a role in the decreased efficacy of the acellular vaccine, compared with the whole-cell vaccine. "The whole-cell vaccine really stimulates a lot more of all different types of immunities," he told Medscape Medical News.

Since the 1990s, acellular pertussis vaccines replaced whole-cell vaccines in the United States, starting with the fourth and fifth doses, then moving to priming doses. Many states mandate the fifth dose for school entry. It is generally given to children 4 to 6 years of age.

The study seems to explain why pertussis has been increasing over the past 25 years, Dr. Baxter said. He explained that researchers have noticed blips every few years that seem to be getting bigger each year, "and it does seem to be happening since we moved to the acellular pertussis vaccine."

He characterized DTaP as both wonderful and problematic. "They're just not as good as the whole-cell vaccines," he said. But part of the problem with the latter is that parents don't want to return to a vaccine that could have severe adverse effects.

What's more, it's hard to get a good read on how effective the acellular vaccine is, the investigator said. When DTaP showed up, pertussis had been wiped out because whole cell "is that good," Dr. Baxter said. There wasn't a way to study how well DTaP vaccines really work, other than by determining whether they produced antibodies to certain antigens. Although they work as well or better at actually producing antibodies, that didn't mean they work as well, he observed.

Myron Levine, MD, DPTH, from University of Maryland School of Medicine in Baltimore, who moderated the presentation, had a different take. The researchers "did not take into account the severity of the disease," Dr. Levine told Medscape Medical News, "and the efficacy is related to the severity of disease." The acellular pertussis vaccine is more protective the more severe the disease gets, he explained.

When asked to clarify context, Dr. Baxter said: "In this case, we're simply using a test and only saying this is how well it protects vs actual tests of positivity."

Dr. Levine later told Medscape Medical News that whole-cell vaccines, which are still used routinely in the developing world, were unfairly attacked in the past. "We're seeing some of the downside of getting rid of the old whole-cell vaccine," but it's impossible to go back to whole cell, because of its detractors who impugned the vaccine for causing severe reactions, he said. "It really turns out that there's no difference between acellular and the old whole cell for those rare severe events," he said.

There's a longer-term effect to moving away from the old whole-cell vaccine, Dr. Levine warned. "What one loses is the adjuvant effects of the whole-cell vaccine," he explained, "and, I think, the more potent stimulation of immunity."

This study was funded internally by the Kaiser Permanente Vaccine Study Center. Dr. Baxter reports receiving research grants from GSK and Sanofi Pasteur. Dr. Levine reports serving as an advisor or consultant for Merck, Aus Bio Ltd, Avant Immunotherapeutics, Medicago, Variation Biotechnologies, Alpha Vax, and Crucell Holland BV; receiving grants for clinical research from Merck; and holding a patent for and/or receiving royalties from an Aventis product.

National Foundation for Infectious Diseases (NFID) 14th Annual Conference on Vaccine Research: Session S22. Presented May 18, 2011.

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