The whooping cough, or pertussis, vaccine is given in five doses - usually before kids start kindergarten.
When researchers from Kaiser Permanente in California compared data on nearly 10,000 children, they found protection from the disease waned after the last dose was given around age 5.
Protection decreased by more than 40 percent every year.
Researchers suggest experts may need to reformulate the current vaccine.
This study period included the large pertussis outbreak on the west coast during 2010.
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