Whooping cough is a tough disease to vaccinate against. Children are given 5 shots before they are old enough to go to school, yet even this multistep regime is failing to protect everyone. Last year a preteen booster shot was added to the vaccination plan in response to rising incidence.
The Centers for Disease Control and Prevention data show U.S. whooping cough rates rising since 1980, with the uptick accelerating over the past decade. In 2010 the number of cases per 100,000 people passed the previous peaks of 2004 and 2005. And--once again--the number of cases in 7- to 10-year-olds was high. In 2007 this age group accounted for 13% of whooping cough cases. By 2009 the figure was up to 23%.
Now a study in the journal Pediatrics has added to evidence that this age bracket--which should be protected by the 5 shots they had as infants--is vulnerable because immunity wanes quickly. Among kids born in Minnesota between 1998 and 2003, 15.6 out of every 100,000 got whooping cough in the first year after vaccination. In the sixth year after the vaccination, the number rose to 138.4 cases. Oregon, the other state analyzed, saw a similar, but less pronounced, pattern.
Observers have suggested various explanations--such as increased physician recognition and improved lab tests--for the rise in overall whooping cough cases. But the increase among 7- to 10-year-olds suggests there is another factor specific to this age group. The explanation given in Pediatrics is that the switch from whole-cell to acellular vaccines in 1997 reduced the duration of immunity. This would explain why incidence in the age group rose in the second half of the last decade, when children who only ever received acellular vaccines began turning 7 years old.
Whatever the explanation, acellular vaccines are here to stay. Whole-cell vaccines were phased out because of safety concerns and no new formulations are imminent. The strategy for now, the researchers say, is making sure as many children as possible receive the acellular vaccine.
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