The acellular vaccine against whooping cough appears to become less effective as the time from the last childhood vaccination increases, researchers reported.
In a large case-control study, children with pertussis were significantly less likely to have had all five doses of the DTaP (diphtheria, tetanus, and acellular pertussis) vaccine than were controls, according to Lara Misegades, PhD, of the CDC, and colleagues.
But even if they had all five doses, vaccine effectiveness was highest in those who were just a year from the final dose and lowest in those who were 5 or more years away, Misegades and colleagues reported in the Nov. 28 issue of the Journal of the American Medical Association.
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, they 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 stated.
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 pertussis and 2,016 controls.
The primary outcomes of the analysis were the odds ratios for the association between pertussis and getting all five doses of DTaP series, and for the association between pertussis and time since finishing the series.
From the odds ratios, they calculated an estimated vaccine effectiveness, defined as 1-OR x 100%.
Misegades and colleagues found that 53 cases (7.8%) and 19 controls (0.9%) had not received any whooping cough vaccine.
Compared with controls, children with pertussis had lower odds of having received all doses of DTaP (OR 0.11, 95% CI 0.06 to 0.21). The corresponding vaccine effectiveness was 88.7%.
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 (OR 0.02, 95% CI 0.01 to 0.04). The vaccine effectiveness was 98.1%.
Finally, the odds ratios increased with each extra year, reaching 0.29 at 60 months or longer, while the corresponding vaccine effectiveness fell, reaching 71.2% at the same time.
The study adds to the "growing consensus" that the acellular vaccines work less well than the earlier whole-cell vaccines, according to Eugene Shapiro, MD, 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.
Primary source: Journal of the American Medical Association
Source reference: Misegades LK, et al "Association of childhood pertussis with receipt of 5 doses of pertussis vaccine by time since last vaccine dose, California, 2010" JAMA 2012; 308: 2126-2132.
Additional source: Journal of the American Medical Association
Shapiro ED "Acellular vaccines and resurgence of pertussis" JAMA 2012; 308: 2149-2150.