Flu pandemic? U.S. has been there for weeks

* More than 1,000 Americans hospitalized with new H1N1 flu

* People under 24 worst affected

* US has "lessons learned" to share

By Maggie Fox, Health and Science Editor

WASHINGTON, June 11 (Reuters) - The World Health Organization may have just declared a pandemic of the H1N1 flu virus, but the United States has been acting as if a pandemic was under way for weeks, health officials said on Thursday.

The new swine flu virus was first identified in two U.S. children in April and by the time the news was out, it had already begun spreading. CDC experts estimate that hundreds of thousands of people are likely infected in every state.

Although most had mild symptoms, there have been at least 27 deaths and 1,000 people have been sick enough to be hospitalized.

Dr. Thomas Frieden, who has been in his job as CDC director for just four days, told a news conference the WHO declaration did not come as a surprise. "There are no practical implications for most state and local governments," he added.

"Here in the United States, the basic declaration isn't going to change our day-to-day activities," added the CDC's Dr. Anne Schuchat.

WHO raised its pandemic flu alert to phase 6 on a six-point scale, indicating the first influenza pandemic since 1968 is under way. [ID:nLB765857]

Based on the U.S. experience, what can the rest of the world expect?

The new H1N1 disproportionately makes younger people sick. Schuchat said 57 percent of U.S. cases were among people aged 5 to 24, and 41 percent of those hospitalized were in this younger age group.

"The highest rates of hospitalization are actually in children under 5," Schuchat said. Pregnant women, people with asthma or other chronic respiratory disease and diabetics needed to take special care and call a doctor if they had symptoms.

H1N1 is active in all 50 states and there are so many cases now that in some areas, patients with specific flu-like symptoms -- a fever above 104 degrees F, cough or other respiratory symptoms -- are presumed to have the new virus.

YEARS OF PLANNING

The United States, like other countries, has been planning for an influenza pandemic for years.