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NEW YORK (AP) - Researchers have zeroed in on a possible explanation for the biggest whooping cough outbreak in decades.

More than 26,000 cases of whooping cough have been reported across the country so far this year, including more than 10,000 in children ages 7 to 10.

Experts say a safer vaccine introduced in the 1990s loses effectiveness much faster than previously thought. A study published in today's New England Journal of Medicine finds that the protective effect weakens dramatically soon after a youngster gets the last of the five recommended shots around age 6.

Researchers at the Kaiser Permanente Vaccine Research Center say the protection rate falls from about 95 percent to 71 percent within five years.

Health officials are considering a number of possible recommendations in response, including another booster shot for children, strengthening the vaccine or developing a new vaccine.

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