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

California schools have turned away thousands of middle and high school students this fall who haven’t gotten the booster shot typically given at age 11 or 12. That state had a huge spike in whooping cough cases last year, during which more than 9,100 people were sickened and 10 babies died after exposure from adults or older children.

The study of cases in Marin County, Calif., 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.

The findings may help explain why significant numbers of fully immunized children got whooping cough in the recent outbreaks.

“I was disturbed to find maybe we had a little more confidence in the vaccine than it might deserve,” said the lead researcher, Dr. David Witt. He is chief of infectious disease at the Kaiser Permanente Medical Center in San Rafael.

Witt presented his findings Monday at an infectious diseases medical conference in Chicago.

Whooping cough is very contagious and in rare cases can be fatal, especially for babies too young to be vaccinated. The disease starts like a cold but leads to severe coughing that can last for weeks.

It also is considered one of the hardest-to-control bacterial illnesses for which a common childhood vaccine is available. Health officials say the vaccine is effective in most people, and yet there are periodic outbreaks in places with high vaccination rates.

More than 80 percent of the children who developed whooping cough in Witt’s study were fully vaccinated.

Young children are recommended to get five doses of a vaccine against whooping cough — at 2, 4, 6 and 15-18 months, and then one more between ages 4 and 6.

The new study found that younger kids who got the fifth dose less than three years before being tested were much less likely to get whooping cough than slightly older kids who were more than three years past their last vaccination.

The study observed what was happening during the California outbreak. It is based on a review last year of roughly 15,000 children in Marin County, including 132 who got whooping cough.

Health officials have acknowledged the long-term effectiveness of whooping cough vaccine is not well understood. The nation switched over to a new type of childhood whooping cough vaccine in the late 1990s, one deemed safer than the version used for decades before.

Short-term effectiveness of the new vaccine has been shown to be 90 percent or greater. But the long-term effectiveness of the childhood vaccine has not been studied as much.

A preliminary study by the Centers for Disease Control and Prevention, conducted last year, found it ebbs. The five doses for young children were about 70 percent effective five years after the last shot.

Witt found that rates of whooping cough — also known as pertussis — dropped dramatically after kids were age 11 and 12, when many get the recommended booster shot.

But the long-term effectiveness of that booster also is not known and has received relatively little study.

“It’s a little too soon to say much” about the longer-term effectiveness of that booster, said Lara Misegades, a CDC epidemiologist who has been studying how well whooping cough vaccines work.

California health officials last year told doctors they could give the booster to kids as young as 7 in an effort to stifle the outbreaks.

CDC officials say that it’s too soon to say if the booster should regularly be given to children that young, but that they are studying the issue.