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 pharmaceutical Co's were especially excited as they stand to make billions on vaccines.

WHO's 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 (12:00 p.m. EDT).

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.