WASHINGTON (Reuters) - Cities that quickly closed schoolsand discouraged public gatherings had fewer deaths from thegreat flu pandemic in 1918 than cities that did not,researchers reported on Monday.

Decisive, immediate action can reduce the most acuteeffects of a pandemic, while allowing the population to buildsome natural immunity to the virus, the U.S. government studyfound.

Experts agree that a pandemic of some virus, most likelyinfluenza, is almost 100 percent certain. What is not certainis when it will strike and which virus it will be.

The worst-case scenario is the 1918 flu pandemic, whichkilled tens of millions of people globally. Researchers aregoing through records to learn from the actions taken decadesago.

Dr. Richard Hatchett of the National Institute of Allergyand Infectious Diseases and colleagues studied 17 U.S. cities.

"Cities in which multiple interventions were implemented atan early phase of the epidemic had peak death rates about 50percent lower than those that did not," they wrote in theirreport, published in the Proceedings of the

National Academy ofSciences.

In Kansas City, no more than 20 people could attendweddings or funerals. New York mandated staggered shifts atfactories. In Seattle, the mayor told people to wear facemasks.

No single action worked on its own, the researchers found,it was the combination of measures that saved lives.

The

World Health Organization has been urging countries toget ready for a pandemic. The H5N1 avian influenza isconsidered the most likely candidate to cause one.

So far, it has spread mostly in birds, across Asia and intoAfrica and Europe. But it sometimes infects people and haskilled 170 people out of 288 known to have been infected.

LESSONS FROM PAST

It is constantly evolving and if it mutates in just theright way, it could spread easily from one person to another,causing widespread death and sickness.

No good vaccine would be available for months, and drugsthat treat influenza are in very short supply.

So experts are looking at what they call non-pharmacologicinterventions -- ways to prevent infection without drugs. Theyhope this can buy time while companies make and distributevaccines and drugs.

Because the virus is spread by small droplets passed withinabout three feet (1 meter) from person to person, keepingpeople apart is considered a possible strategy.

Hatchett's study suggests this worked nearly a century ago.

"The first cases of disease among civilians in Philadelphiawere reported on September 17, 1918, but authorities downplayedtheir significance and allowed large public gatherings, notablya citywide parade on September 28, 1918, to continue," theywrote.

"School closure, bans on public gatherings, and othersocial distancing interventions were not implemented untilOctober 3, when disease spread had already begun to overwhelmlocal medical and public health resources," they added.

St. Louis authorities introduced "a broad series ofmeasures designed to promote social distancing" as soon as flushowed up.

Philadelphia ended up with a peak death rate of 257 peopleper 100,000 population per week. St. Louis had just 31 per100,000 at the peak.

When the cities relaxed their policies, death rates rose.

The U.S. government flu plan calls for similar measures,including allowing employees to stay home for weeks or evenmonths, telecommuting and closing schools and perhaps largeoffice buildings.