The number of confirmed A/H1N1 flu cases in the southern U.S. state of Florida has surged from two to 23 in the past week as the Centers for Disease Control and Prevention (CDC) enabled states to test their own samples, clearing backlogs, a local health official said on Friday.

Last Friday, Governor Charlie Crist announced a 17-year-old girl and an 11-year-old boy as the first two confirmed cases of A/H1N1 flu, commonly known as swine flu, in the state.

Nationwide, the number of sick nearly doubled from Thursday to Friday, reaching 1,639, in 43 states. The Florida Department of Health said new test kits from the CDC were being used at its four state labs, which are in Miami, Tampa, Jacksonville and Pensacola.

Tim O'Connor, health department spokesman, said that positive cases were confirmed in 11 counties, including Broward, Indian River and Okeechobee. While Palm Beach County wasn't among those with confirmed A/H1N1 influenza cases, it had one probable case, he said, adding that individual has been treated and has already recovered.

The A/H1N1 flu strain is different to seasonal flu in key ways, the CDC reported in Friday's edition of its scientific journal, Morbidity and Mortality Weekly Report.

It's hitting young people hardest, rather than the very old and very young who are typically cut down by seasonal flu, the report said, adding that it's rising during what's typically the off season for flu.

"In Mexico and the United States, the percentage of patients requiring hospitalization has been particularly high among persons aged 30-44 years," the journal editors wrote.

Local health officials said that Florida's flu cases mirror the national data -- the young, especially teenagers, have been hardest hit.

Broward County's 5 confirmed positive cases were between the ages of 3 and 26, while Indian River's case was in a 15-year-old, and Okeechobee's was in a 27-year-old, they explained.

(Xinhua News Agency May 9, 2009)