NEW YORK (AP) — 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.