* WHO chief Margaret Chan raises pandemic alert level to 5
* Shift encourages drug stockpiling, vaccine development
(Adds quotes from Chan and Fukuda)
GENEVA, April 29 (Reuters) - The World Health Organisation on Wednesday raised the global flu alert level to 5, the second highest phase that indicates "a pandemic is imminent".
"I have decided to raise the level of influenza pandemic alert from phase 4 to phase 5," WHO Director-General Margaret Chan told a news briefing about the swine flu outbreak that has been deadly in Mexico and the United States and which has caused milder symptoms elsewhere.
"The world is better prepared for an influenza pandemic than at any time in history," Chan said. "For the first time in history we can track the pandemic in real time."
In Mexico up to 159 people have died and about 1,300 more are being tested. In the United States a 22-month-old boy has died in Texas while on a visit from Mexico.
According to the WHO's pandemic flu response guidelines, a Phase 5 alert is called when there is sustained human-to-human spread of the virus in at least two countries in one region.
"While most countries will not be affected at this stage, the declaration of Phase 5 is a strong signal that a pandemic is imminent and that the time to finalise the organisation, communication and implementation of the planned mitigation measures is short," the document reads.
Chan's move followed intensive consultations with experts and analysis of the spreading virus within and from Mexico.
The change in level puts governments on alert about the need to stockpile antiviral drugs such as Roche Holding AG (ROG.VX)'s Tamiflu and GlaxoSmithKline's (GSK.L) Relenza, and accelerates pharmaceutical industry efforts to create a vaccine to fight the swine flu strain.
The higher alert also signals the virus is unlikely to fade away, and could pose a prolonged public health threat.
"It is something here to stay. Now the question is at what level it will be circulating and how easily it is transmitted," one public health expert said before the official announcement.
Keiji Fukuda, WHO acting assistant director-general, said swine flu was spreading in a "convincing" way in both Mexico and the United States and would likely permeate other countries and regions in a similar way.
"By definition a pandemic is when we get to Phase 6 but we think we are in the process of moving towards there," he told the late evening press conference at the WHO headquarters.
On Monday evening, Chan convened a meeting of the WHO's emergency committee to assess the pandemic alert level which had previously been at 3 on the six-stage scale.
That panel recommended an increase to 4, which formally indicated the infection could spread between humans to cause "community-level outbreaks".
Chan said because all flu viruses were unpredictable, the current swine flu strain needs to be closely watched in case it mutates into a more dangerous strain -- or a milder one.
"The important point for us now to continue to maintain our vigilance and track its movement," she said.
The WHO is not recommending travel restrictions, border closures, or any limitation on the movement of people, goods or services as a result of the outbreak.
Chan also stressed there was no related danger from eating properly-cooked pork. She recommended good hygiene measures such as hand-washing to prevent infection from flu and said it was important "to maintain a level of calm". (For a NEWSMAKER on Margaret Chan, click on [LS818002]) (For more Reuters stories on swine flu, click on [nFLU]) (For more Reuters swine flu coverage, please click here: here ) (For WHO information on swine flu, go to: here )