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 U.S. 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.

All adults, including pregnant women, should get a booster shot because the contagious illness is especially dangerous for children under a year old who have yet to complete a cycle of vaccinations, Dr. Anne Schuchat, director of the CDC's National Center for Immunization and Respiratory Diseases, told a media briefing.

"It's most dangerous for babies," Schuchat said, adding that the current outbreak at its existing pace could become the most severe since 1959, when 40,000 cases were reported. "Preventing infant deaths from the disease is our primary national goal," she said.

A spike in whooping cough cases among 10-year-olds and adolescents who are 13 and 14 was a concern, perhaps an indicator that the pertussis vaccine may be wearing off earlier than anticipated, Washington Health Secretary Mary Selecky said.

The U.S. groups of 10-, 13- and 14-year-olds who are experiencing a high illness rate had DTaP vaccinations, which were introduced in 1997 at the same time that the prior DTP vaccine was discontinued.

The earlier vaccine used whole cell parts made of killed pertussis bacteria, while DTaP uses only small acellular bacteria pieces, not the whole bacteria cell, said Donn Moyer, Washington state Health Department spokesman

CDC officials will begin an investigation in Washington state later this month "to analyze our data for cases among 13- to 14-year-olds to see what can be learned about disease rates and vaccination status," Moyer told Reuters.

The number of cases in Washington, with no deaths, has tripled since April, with 1,132 cases reported by the end of that month.

Moyer said the CDC also plans a similar study in California, where a 2010 epidemic counted more than 9,000 cases, including 10 infant deaths.

Whooping cough typically begins with cold-like symptoms such as a fever, runny nose and sneezing and is accompanied by a mild cough that grows more severe by the first or second week. A high-pitched whoop, giving the illness its name, can follow violent coughing fits.

This year, Australia is experiencing a high rate of whooping cough, Schuchat said.

(Editing by Cynthia Johnston and Philip Barbara)

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