From the Associated Press:

The U.S. appears headed for its worst year for whooping cough in more than five decades, with the number of cases rising at an epidemic rate that experts say may reflect a problem with the effectiveness of the vaccine.

Nearly 18,000 cases have been reported so far – more than twice the number seen at this point last year, the Centers for Disease Control and Prevention said Thursday. At this pace, the number for the entire year will be the highest since 1959, when 40,000 illnesses were reported.

Nine children have died, and health officials called on adults – especially pregnant women and those who spend time around children – to get a booster shot as soon as possible.

“My biggest concern is for the babies. They’re the ones who get hit the hardest,” said Mary Selecky, chief of the health department in Washington, one of the states with the biggest outbreaks. Washington and Wisconsin have reported more than 3,000 cases each, and high numbers have been seen in a number of other states, including New York, Minnesota and Arizona.