GENEVA - The World Health Organization was poised on Thursday to declare that the new H1N1 virus has caused the first influenza pandemic in more than 40 years, health sources said on Thursday.

The move will trigger heightened health measures in the WHO's 193 member states as authorities brace for the worldwide spread of the virus that has so far caused mainly mild illness.

WHO Director-General Dr Margaret Chan was to hold a news conference on the outbreak at 1600 GMT.

Flu experts advising Chan, who met earlier on Thursday, were expected to recommend moving to the top phase 6 on the WHO's six-point scale, the sources said.

That would reflect the fact that the disease, widely known as swine flu, was spreading geographically, but not necessarily indicate how virulent it is.

Phase 6, if we call a phase 6, doesn't mean anything concerning severity, it is concerning geographic spread ... Pandemic means global, but it doesn't have any connotation of severity or mildness, WHO spokesman Gregory Hartl said.

In fact, what we are seeing with this virus so far is overwhelmingly to date mild disease. So we would think that this event is really a moderate event for the time being, because the numbers are high but the disease is overwhelmingly mild, he told Reuters Television before the talks.

David Heymann, a former top WHO official now chairing Britain's Health Protection Agency, said that countries had tried to contain the virus through measures including school closures during the current phase 5. This has extended the precious time needed to prepare for a full-blown pandemic.

During phase 5, the government and people in the U.K. have had the time to prepare for a pandemic -- this has hopefully decreased any surprise and concern that might be associated with a WHO announcement of phase 6, if one is made, he told Reuters.

As it spreads in humans, science cannot predict what course the virus will take, the disease it causes and the age groups infected, Heymann said. The severity of that disease, the effectiveness of antiviral drugs and the stability of the virus must all be watched closely, he added.

A pandemic could cause enormous disruption to business as workers stay home because they are sick or to look after family members and authorities restrict gatherings of large numbers of people or movement of people or goods.

World markets shrugged off the possibility of a pandemic, as investors focused on possible global economic recovery.

AUSTRALIA LIKELY TRIGGER

Widespread transmission of the virus in Victoria, Australia, signaling that it is entrenched in another region besides North America, is likely to be the trigger for moving to phase 6.

Five people have been admitted to intensive care in Australia and more than 1,000 cases confirmed following widespread testing in the state.

We have tested 5,500 people in the last two weeks, that is more people than we test in our whole influenza season, said Victorian state premier John Brumby.

One health source, who declined to be named, said the experts were also expected to recommend finishing production currently under way of seasonal flu vaccine for the northern hemisphere next winter.

They might say finish seasonal vaccine and say begin pandemic vaccine as soon as it is feasible, he said.

Drugmakers have obtained the new influenza A (H1N1) seed virus in the past two weeks, enabling them to begin the production process by growing the virus in eggs.

Company officials said on Wednesday that they were on track to have a vaccine against the new strain ready for the northern hemisphere autumn.

Seasonal flu each year kills up to half a million people, mainly elderly, and causes severe illness in millions, so a premature switch in vaccine production to cope with the new strain could put many people at risk.

The new strain can be treated by antiviral drugs oseltamivir, the generic name of Roche Holding's Tamiflu tablets, and Relenza, a spray made by GlaxoSmithKline.

The strain, which emerged in April in Mexico and the United States, has spread widely in nations including Australia, Britain, Chile and Japan.

Authorities in Germany have confirmed 30 cases of H1N1 at a school in the industrial Rhineland city of Duesseldorf, the most concentrated outbreak of the virus so far in Europe's biggest economy.

There have been 27,737 infections reported in 74 countries to date, including 141 deaths, according to the WHO's latest tally of laboratory confirmed cases, but the real number of people with the disease is likely to run into at least hundreds of thousands, as mild cases may not have been detected.

A survey by New York City's health department showed that 6.9 percent of the city's population of over 8 million had experienced flu-like illness -- which could include other diseases -- in the first three weeks of May.

The findings don't tell us exactly how many New Yorkers have had H1N1 influenza, said New York City Health Commissioner Dr Thomas Farley in a statement. But they suggest it has been widespread, and mild in most affected people.

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