Sara Y. Tartof and Melissa Lewis of the Centers for Disease Control and Prevention, Melissa Lewis of the Minnesota Department of Health, Dr. Juventila Liko of the Oregon Health Authority and colleagues assessed the risk of pertussis after children in Minnesota and Oregon received five doses of acellular pertussis vaccines -- the DTaP vaccine.
The researchers tracked 224,378 Minnesota children and 179,011 Oregon children -- born from Jan. 1, 1998, to Dec. 31, 2005, who had all five doses of the DTaP vaccine.
The study, published in the journal Pediatrics, found 458 case of whooping cough were confirmed by a doctor in Minnesota and 89 pertussis cases were identified in Oregon -- and pertussis incidence rates rose each year of follow-up.
In Minnesota, the whooping cough cases rose from 15.6 per 100,000 the first year to 138.4 per 100,000 people in year six. For Oregon the pertussis incidence was 6.2 per 100,000 in year 1 and 24.4 per 100,000 in year six.
The study said the duration of protection from the whooping cough vaccine in children with five doses of DTaP might wane earlier than expected.