In today's Health matters, the whooping cough vaccine appears to fade after about three years.
Researchers found children who completed their vaccinations three or more years ago were 20 times more likely to come down with the disease.
On the other hand, kids who were vaccinated more recently appeared to be better protected.
Medical experts say the findings may help explain why large numbers of fully immunized children got whooping cough in recent outbreaks.
California schools turned away thousands of students this fall who did not have the whooping cough booster shot.
The vaccination is typically to 11 and 12-year-olds. California had a huge spike in whooping cough cases last year where more than 9,000 people became ill. And ten babies died.