NEW YORK — As the U.S. wrestles with its biggest whooping cough outbreak in decades, researchers appear to have zeroed in on the main cause: The safer vaccine that was introduced in the 1990s loses effectiveness much faster than previously thought.

A study published in Wednesday's New England Journal of Medicine found that the protective effect weakens dramatically soon after a youngster gets the last of the five recommended shots around age 6.

The protection rate falls from about 95 percent to 71 percent within five years, said researchers at the Kaiser Permanente Vaccine Research Center in Oakland, Calif.