ATLANTA (AP) -- A preliminary study suggests the vaccine against whooping cough falters after only about three years, adding support to school rules requiring kids to get the vaccination periodically.

California schools have turned away thousands of students this fall who aren't up to date on their shots. That state had a huge spike in whooping cough cases last year, and 10 babies died after exposure from adults or older children.

The study in one California county found the risk of getting the disease was as much as 20 times higher in kids three years or more after they finished receiving a recommended series of vaccinations. But kids vaccinated more recently were well protected.

Dr. David Witt, the lead researcher, presented the findings Monday at an infectious diseases medical conference in Chicago.