WASHINGTON, June 24 (Reuters) - Half a million Americans could die and more than 2 million could end up in the hospital with serious complications if an even moderately severe strain of a pandemic flu hits, a report predicted on Friday.

But the United States only has 965,256 staffed hospital beds, said the report from the Trust for America´s Health.

The non-profit group´s state-by-state analysis adds to a growing clamor of voices contending that the United States is not prepared for a large outbreak of disease, whether natural or brought on by war or terrorism.

"This is not a drill. This is not a planning exercise. This is for real," said the Trust´s executive director, Shelley Hearne, in a statement.

In an average year, influenza kills an estimated 36,000 Americans and puts 200,000 into the hospital.

A more serious strain strikes every few years and a so-called pandemic strain emerges once every 27 years, on average. The more virulent strains sweep around the world within months.

Pandemics hit in 1918 -- killing up to 40 million people globally -- 1957 and 1968. Health experts all say the world is overdue for another and fear the avian flu in Asia may be it.

The World Health Organization says an H5N1 avian flu pandemic could kill up to 7.4 million people globally, because people lack immunity to it.

Avian flu has not yet acquired the ability to pass easily from person to person, but would spread rapidly if it does, experts say.

PANDEMIC RISKS

But even another strain of flu could wreak havoc if it has pandemic characteristics, according to the report.

"The U.S. has not adequately planned for the disruption a flu pandemic could cause to the economy, daily life, food and supply distributions, or homeland security," the Trust´s report reads.

"The U.S. lags in pandemic preparations compared to Great Britain and Canada based on an examination of leadership, vaccine development, vaccine and antiviral planning, health care system surge capacity planning, coordination between public and private sectors, and emergency communications planning."

Health officials are aware of the warnings and say they are developing a plan. The heads of the Centers for Disease Control and Prevention, National Institutes of Health and Health and Human Services Department are regularly called to hearings before Congress on the issue.

The Trust said more could be done to prepare now, including setting up plans to track an outbreak, stockpile antiviral drugs and other medical supplies and set up communications.

The study found that the United States has stockpiled 2.3 million courses of the best anti-influenza drug, oseltamivir, marketed by Gilead Sciences <GILD.O> and Roche <ROG.VX> under the brand name Tamiflu.

It has placed orders for 3 million more courses of the drug, which does not cure influenza but can prevent infection if taken early enough and can reduce its severity.