The vaccine against whooping cough will weaken after only about three years, according to a new U.S. study, which lends support to school rules requiring periodic boosters for children.

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

The study was done in California, where whooping cough vaccinations are a critical issue. The state had an increase in whooping cough cases last year, during which more than 9,100 people fell ill and 10 babies died.

Witt's study looked at roughly 15,000 children in Marin County, including 132 who got whooping cough last year. The study found the risk of disease was as much as 20 times higher in children three years or older after they finished receiving a recommended series of vaccinations. But children vaccinated more recently were well protected.

California schools have turned away thousands of middle and high school students this fall who haven’t received the booster shot, typically given at age 11 or 12.

Whooping cough is a highly contagious bacterial disease caused by Bordetella pertussis. Symptoms are initially mild but develop into severe coughing fits, which produce the namesake high-pitched whoop sound in infected babies and children when they inhale air after coughing.

The incubation period is typically seven to 10 days in infants or young children, after which there are usually mild respiratory symptoms, mild coughing, sneezing, or runny nose. This is known as the catarrhal stage. After one to two weeks, the coughing classically develops into uncontrollable fits, each with five to 10 forceful coughs, followed by a high-pitched whoop sound in younger children, or a gasping sound in older children, as the patient struggles to inhale.

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