SEATTLE - The number of US whooping cough cases has risen to around 18,000 in an outbreak that is on track to become the most severe in over a half century and could in part stem from possible waning vaccine protection, health officials said on Thursday.

Washington state, which declared an epidemic in April, and Wisconsin were particularly hard hit, with each reporting more than 3,000 cases, according to the US Centers for Disease Control and Prevention.

Nine people have died overall and the number of cases was already more than double than at the same time last year.

Whooping cough, also known as pertussis, causes severe, almost uncontrollable coughing. In the United States, most children are immunized with a DTaP five-dose series vaccine that is given as a series of shots, starting at two months.

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