The vaccine against whooping cough appears to become less effective as the time from the last childhood vaccination increases, a new study found.
The CDC study of nearly 2,700 children found those with whooping cough were less likely to have had all five doses of the DTaP (diphtheria, tetanus, and acellular pertussis) vaccine than those without the infection.
But even in children who had all five doses, the vaccine's effectiveness was highest in those who were just a year from the final dose and lowest in those who were five or more years away, study author Lara Misegades and colleagues reported in the Nov. 28 issue of the Journal of the American Medical Association.
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The findings, based on the 2010 whopping cough outbreak in California, suggest a need for review of vaccine programs and perhaps even development of a new vaccine, the authors argued.
The initial introduction of a whooping cough vaccine in the 1940s, based on whole-cell killed Bordetella pertussis, dramatically cut the incidence of the disease. But the whole-cell vaccine was associated with a host of adverse effects. In the 1990s, an acellular version was introduced, in combination with diphtheria and tetanus toxoids.
Since then, in several outbreaks, observers have noted that children ages 7 to 10 appear to be less well protected than younger children. In the 2010 California outbreak, the rate of whooping cough in that age group was 67.9 cases per 100,000, the authors wrote.
The observations have suggested that the vaccine's effectiveness falls off over time. To test that idea, the researchers studied the vaccine history of 2,698 children in 15 California counties during the 2010 outbreak, including 682 who developed whooping cough and 2,016 controls.
Fifty-three cases (7.8 percent) and 19 controls (0.9 percent) had not received any whooping cough vaccine. Compared with controls, children with whooping cough had lower odds of having received all doses of DTaP. The corresponding vaccine effectiveness was 88.7 percent.
When children were categorized by time since finishing the series, children with pertussis were less likely than controls to have received their fifth dose within the previous 12 months with a vaccihe vaccine effectiveness of 98.1 percent. But after 60 months or longer, the vaccine effectiveness fell to 71.2 percent.
The study adds to the "growing consensus" that the acellular vaccines work less well than the earlier whole-cell vaccines, according to Dr. Eugene Shapiro of Yale University School of Medicine.
But the cause of the difference remains unclear, he argued in an accompanying editorial. Explanations range from earlier waning of immunogenicity, as the study authors suggest, to genetic changes in the pathogen.
While acellular vaccines might be less than perfect, they are still effective, Shapiro argued. He stressed the importance of using "the current vaccines to provide optimal benefit," especially for infants, who are the most vulnerable to whooping cough.
Exactly how that is to be done, Shapiro argued, will need further study, including looking at different immunization schedules and target populations, changes in booster timing, and perhaps even a partial return to whole-cell vaccines.