ATLANTA — A government study offers a new theory on why the whooping cough vaccine doesn't seem to be working as well as expected.

Researchers concluded that while the shot may keep people from getting sick, it doesn't prevent them from spreading the germ.

Whooping cough is a highly contagious disease most dangerous to children. Last year was the nation's worst year for whooping cough in six decades. Some studies have suggested that a newer version of the vaccine doesn't last as long as the old one.

The new research tested the vaccine in baboons. They didn't get sick but still may have been contagious for five weeks.

The study was published Monday in the journal Proceedings of the National Academy of Sciences.

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