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September 12, 2012
Ted Warren photo

FILE - In this Thursday, May 3, 2012 file photo, nurses Fatima Guillen, left, and Fran Wendt, right, give Kimberly Magdeleno, 4, a whooping cough booster shot, as she is held by her mother, Claudia Solorio at a health clinic in Tacoma, Wash. As the U.S. wrestles with its biggest whooping cough outbreak in decades, researchers appear to have zeroed in on the main reason: The safer vaccine that has been in use since the 1990s loses effectiveness much faster than previously thought. A study published in the New England Journal of Medicine on Wednesday, Sept. 12, 2012 found that the protective effect weakens dramatically soon after a youngster gets the last of the five recommended shots around age 6.