MINNEAPOLIS (WCCO) — There have been more than 2,400 confirmed cases of whooping cough in Minnesota this year — and 26,000 cases nationwide.

But a new study is questioning the effectiveness of the whopping cough vaccine.

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The DTaP vaccine protects against diphtheria, tetanus, and pertussis, aka whooping cough. The study raises concerns about the final dose of the vaccine, which is usually given to 4- to 6-year-old children.

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Researchers found the effectiveness of the vaccine fades about 40 percent four years after it is given.

“As time goes on and the vaccines get less and less effective…it’s time for those booster shots,” said Dr. Amy Porter, of the Kaiser Permanente Medical Center.

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Experts recommend older people get a booster shot if they didn’t get one in their teens. In the study, 80 percent of kids who had whooping cough, caught it from an older relative.