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NEW YORK (AP) - Researchers have discovered the first U.S. cases of whooping cough caused by a germ that may be vaccine resistant.

Health officials are looking into whether that might be one reason why the U.S. just had its worst year for whooping cough in six decades. The new bug was previously reported in Japan, France and Finland.

The U.S. cases are detailed in a brief report in Thursday's New England Journal of Medicine.

Whooping cough used to be common until a vaccine was introduced in the 1940s. An increase in recent years has been partially blamed on a vaccine used since the 1990s, which doesn't last as long.