By: Tartof, Sara Y. ; Lewis, Melissa ; Kenyon, Cynthia
In an effort to determine the risk of pertussis in the years after vaccination, researchers studied more than 400,000 children from Minnesota and Oregon who had five doses of DTaP recorded in state Immunization Information Systems. Out of the 224,378 children in Minnesota and 179,011 in Oregon who were born between 1998 and 2003, there were 458 cases of pertussis in Minnesota and 89 in Oregon. The study shows that pertussis incidence rates rose steadily each year following vaccination. Risk ratios edged up to 8.9 in year six from 1.9 in year two in Minnesota and to 4.0 in year six from 1.3 in year two in Oregon. As additional cohorts receiving only the DTaP vaccine are introduced, the researchers say disease burden and vaccine effectiveness need to be continually monitored. They believe waning immunity from DTaP vaccines could be partially responsible for the increased risk of pertussis.