(HealthDay News) – Children who receive five doses of the acellular pertussis vaccine (DTaP) have an increasing risk of developing the illness in the six years after the last dose, supporting the concept of waning vaccine immunity and possibly explaining the increasing proportion of cases among 7- to 10-year-old children, according to a study published online March 11 in Pediatrics.

Sara Y. Tartof, PhD, MPH, from the U.S. Centers for Disease Control and Prevention in Atlanta, and colleagues examined the incidence and risk of pertussis in 224,378 children in Minnesota and 179,011 children in Oregon born between 1998 and 2003 who received five doses of DTaP. The acellular vaccine was recommended over the whole-cell vaccine for all five doses starting in 1997.

The researchers found that there were 458 cases of pertussis in Minnesota and 89 pertussis cases in Oregon. The incidence of pertussis rose from year one to year six after the fifth dose in both Minnesota (15.6–138.4 per 100,000) and Oregon (6.2–24.4 per 100,000). From year two to year six after the fifth dose, the risk ratios increased from 1.9 to 8.9 in Minnesota and from 1.3 to 4.0 in Oregon.

“This evaluation reports increasing risk of pertussis in the six years after receipt of the fifth DTaP dose, suggesting that waning of vaccine-induced immunity is occurring before the recommended adolescent booster dose at 11 to 12 years of age,” Tartof and colleagues conclude.

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