* WHO declares first flu pandemic in 40 years
* Flu experts recommend move to phase 6
* Global assessment moderate, WHO warns against complacency
* WHO urges countries to finish seasonal vaccine production
* WHO advises against border closures or travel restrictions
(Adds comments from news conference)
By Stephanie Nebehay
GENEVA, June 11 (Reuters) - The World Health Organisation declared an influenza pandemic on Thursday and called on governments to prepare for a long-term battle against an unstoppable flu virus.
The United Nations agency raised its pandemic flu alert to phase 6 on a six-point scale, indicating the first influenza pandemic since 1968 is under way.
"With today's announcement, WHO moves from an emergency to a longer-term response. Based on past experience, this pandemic will be with us for some months, if not years, to come," WHO Director-General Dr Margaret Chan said in a letter to staff, a copy of which was obtained by Reuters.
People aged 30-50, pregnant women or people suffering from chronic conditions such as asthma, diabetes or obesity are deemed at highest risk, she told a news conference.
Countries from Australia to Chile and the United States are also reporting that the new virus, commonly known as swine flu, is "crowding out" seasonal flu, becoming the predominant influenza strain, she said.
For now the virus was "pretty stable", but Chan warned that it could still mutate into a more deadly form, taking on characteristics of the separate H5N1 bird flu virus circulating widely in poultry.
"So it is incumbent on WHO and all members to stay vigilant and alert for the next year or two or even beyond, she said.
WHO's top flu expert Keiji Fukuda agreed, saying: "When you're talking about pandemic influenza, you are talking about a marathon, you are not talking about a sprint."
Acting on the recommendation of flu experts, the WHO reiterated its advice to its 193 member countries not to close borders or impose travel restrictions to halt the movement of people, goods and services, a call echoed by U.N. Secretary-General Ban Ki-moon.
The move to phase 6 reflects the fact that the disease, widely known as swine flu, was spreading geographically, but does not indicate how virulent it is.
Widespread transmission of the virus in Victoria, Australia, signalling that it is entrenched in another region besides North America, was one of the key triggers for moving to phase 6.
NO SURPRISE
"This is not a surprise," Dr Thomas Frieden, new director of the U.S. Centers for Disease Control and Prevention, told a news conference. "It is expected based on the data."
A unanimous experts' decision was based on an overall assessment in the eight most heavily hit countries -- Australia, Britain, Canada, Chile, Japan, Mexico, Spain and the United States -- that the virus is spreading in a sustained way in communities, according to Chan.
"Collectively, looking at that, we are satisfied that this virus is spreading to a number of countries and it is not stoppable," she said.
"Moving to pandemic phase six level does not imply we will see an increase in the number of deaths or very severe cases.
"Quite on the contrary. Many people are having mild disease, they recover without medicines in some cases and it is good news," she said.
And measuring the impact of the disease as it develops and spreads is difficult.
"It is very hard to get a sense of how many people are really dying from something like pandemic flu," Fukuda said.
Chan said the WHO global assessment of the pandemic was that it was moderate, but at the national level the picture could be different.
"Although the pandemic appears to have moderate severity in comparatively well-off countries, it is prudent to anticipate a bleaker picture as the virus spreads to areas with limited resources, poor health care, and a high prevalence of underlying medical problems," she said.
VACCINE PRODUCTION
The WHO recommended drugmakers stay on track to complete production of seasonal influenza vaccine for the northern hemisphere's next winter in the next two weeks. Each year, normal flu kills up to 500,000 people and infects millions.
Seasonal flu affects mainly the 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.
"So our recommendation is they need to finish the seasonal vaccine and then move over. That is pretty good in terms of dove-tailing the production of the two types of vaccine. So I think that this may just work out," Chan said.
Chan said the Geneva-based agency would work with regulatory authorities to help fast-track approval of new pandemic vaccines that are safe and effective so that they can be made available as soon as possible. In any case, the first doses would only be available in September, she 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.
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. There have been 28,774 infections reported in 74 countries to date, including 144 deaths, according to WHO's latest tally of laboratory-confirmed cases. (For the full text of Chan's remarks at a press conference, go to: http://www.who.int/mediacentre/news/statements/2009/h1n1_pandemic_phase6_20090611/en/index.html ) (For more Reuters swine flu coverage, please go to: http://www.reuters.com/news/globalcoverage/swineflu ) (For WHO information on swine flu, go to: http://www.who.int/csr/disease/swineflu/en/index.html ) (Additional reporting by Laura MacInnis in Geneva, Michael Kahn in London, and Maggie Fox in Washington; Editing by Jonathan Lynn and Angus MacSwan)