"This virus travels at an unbelievable, almost unheard of speed," World Health Organisation Director General Margaret Chan told France's Le Monde daily in an interview.

"In six weeks it travels the same distance that other viruses take six months to cover," Chan said.

"Sixty percent of the deaths cover those who have underlying health problems," Chan said. "This means that 40 percent of the fatalities concern young adults -- in good health -- who die of a viral fever in five to seven days.

"This is the most worrying fact," she said, adding that "up to 30 percent of people in densely populated countries risked getting infected."

Chan's warning came a day after the WHO said the virus had overtaken others to become the most prevalent flu strain.

"Evidence from multiple outbreak sites demonstrates that the A(H1N1) pandemic virus has rapidly established itself and is now the dominant influenza strain in most parts of the world," the UN agency said in a statement.

"The pandemic will persist in the coming months as the virus continues to move through susceptible populations," it added.

Chan underlined that emergency and healthcare services in several countries had come under strain and stressed that resources allocated for cancer patients and those suffering from heart disease should not be diverted.

"One must not rob Peter to pay Paul," she said. "All governments must prepare for the worst."

She said the most important thing in the battle against the virus was "political leadership."

More than 2,180 people around the world have died from the virus since it emerged in April, according to the latest WHO figures.

Chan also said that it could be months before sufficient vaccine is available to combat the pandemic.

She put world production capacity at 900 million doses a year, for a global population of 6.8 billion people.

Even if this was an unprecedented effort, and authorities were speeding up procedures for getting vaccines to the market, there should be no question of compromises on their safety and effectiveness, Chan said.

Britain and France received their first batches of swine flu vaccine this week. Australia on Friday said a massive swine flu vaccination programme would start in October and Turkey hopes the first supplies of the vaccine will come by that time.

While 90 percent of severe and fatal cases occur in people aged above 65 in seasonal flu, most of those who die from swine flu are under the age of 50.

A "very severe form of disease" affecting the lungs and causing severe respiratory failure among young and healthy people was being reported, WHO said Friday, adding that highly specialised care was required.

Large numbers of such patients could therefore "overwhelm" intensive care units and disrupt the provision of care for other diseases, it warned.

In the southern hemisphere where the flu-prone winter season is tailing off, the WHO said cities in several countries had reported that nearly 15 percent of hospitalised cases required intensive care.

earlier related report
When flu viruses 'shift and drift', how many vaccines?
The World Health Organisation's announcement Friday that the 2009 H1N1 virus has become the dominant strain of flu worldwide fits a historical pattern, but the impact on vaccine policy remains unclear, a top expert said.

"This was one of the big questions, whether we would finish up with three types of strains, or whether one would replace the others," said Nigel Dimmock, a virologist and emeritus professor at the University of Warwick.

Dimmock said it was surprising that the WHO had decided so rapidly that the pandemic strain -- which surfaced in Mexico four months ago -- had elbowed out other flu strains.

The northern hemisphere flu season has not yet begun, so the determination must have been based mainly on what has happened in southern hemisphere countries such as Australia, New Zealand, Argentina and Brazil, he said.

The WHO said in a communique: "Evidence from multiple outbreak sites demonstrates that the H1N1 pandemic virus has rapidly established itself and is now the dominant influenza strain in most parts of the world."

Human flu viruses are identified by a surface-lying protein called hemagglutinin -- the "H" of A(H1N1).

There are 16 subtypes of the "H" protein that circulate in birds and swine, but so far only three have become easily transmissible among humans.

"Flu viruses go through 'shift and drift', explained Mark Miller, an epidemiologist at the US National Institutes of Health's Fogarty International Center.

"In previous pandemics we had a shift in the type of virus that is dominant," and then a process of 'drift' and it changed slightly from year-to-year.

The deadly 1918 "Spanish flu" -- which left some 40 million dead -- was an H1 virus, and dominated unchallenged until 1956 when a new "H2" variant swept across the globe.

"It pretty much at a stroke replaced the H1N1. It is still amazing how that happened -- nobody really knows why," Dimmock said.

The same thing happened a dozen years later, when the "H3" knocked its predecessor off the world stage. This led scientists to conclude that any new pandemic strain replaces the seasonal one descendent from the previous pandemic.

"But then the H1N1 reappeared in 1977, very similar to the strains circulating in the mid-1950s, and it has been co-circulating with the H3N2 up until the start of this year," Dimmock said.

It was unclear based on the WHO's statement, he added, whether people would need to vaccinate against only the new swine flu virus, or against the two strains that have been around for decades as well.

"If the pandemic H1N1 has replaced the other strains, as happened in 1957 and 1968, then you don't need the old vaccine," Dimmock said by phone.

"But if they continue to co-circulate, even if it is dominant, then you are going to need a vaccine with three type-A components -- and a seasonal B."

The vaccines can be combined into a single shot, or given separately he said.

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