The United Nations agency’s guidelines state that as soon as the virus starts spreading freely in two regions of the world, its six-point pandemic alert should be raised to the top notch.

With infection numbers rising in Europe, public health experts are struggling to decide whether it is worth sounding the full alarm over H1N1, which is treatable with existing drugs and appears less severe than seasonal flu in most cases.

“It is a judgement call,” one WHO official said when asked about whether the global alert needs to hit its top rung.

WHO Director-General Margaret Chan raised it to level 5 — signalling a pandemic was “imminent” — last week after the flu strain that killed young adults in Mexico emerged in the United States and Canada and spread from schools to communities there.

Under the rules, just one country outside the Americas needs to have a community-level outbreak of the new strain to trigger a Phase 6 designation indicating a global pandemic is under way.

Chan has sought to prepare the public for the declaration of a full pandemic of H1N1, which is widely known as “swine flu” and also contains pieces of human and bird viruses.

“Level 6 does not mean, in any way, that we are facing the end of the world,” she told the Spanish daily El Pais this week. She stressed that the alert ladder indicates how likely the virus is to spread around the world, not how dangerous it is.

The WHO’s recommendations about how to respond to a pandemic are virtually the same for alert levels 5 and 6.

In both cases, countries with outbreaks are told to consider closing schools and cancelling public events, and to distribute drugs and procure vaccines as means allow, while “countries not yet affected” are urged to prepare themselves for the virus.

“The best way would be to describe them as a continuum of actions that countries will take at different points of time, depending on their particular circumstances,” WHO spokesman Thomas Abraham said of the top two alert phases.

BRITAIN AND SPAIN IN FOCUS

The H1N1 flu has hit Mexico hardest, killing 42 people there and infecting nearly 1,000.

While Mexican authorities have said their outbreak has peaked, raising hopes the public health scare is over, officials at the WHO are continuing to pore over data about the spread of the virus around the globe.

Almost all the people with confirmed infections outside the Americas region to date had travelled to Mexico or had close contact with people who did.

But some experts believe Britain is especially vulnerable to a community-wide outbreak, given the flu strain is propagating inside some schools. Spain also has a relatively large cluster of infections, with 73 according to the latest WHO tally.

Chan, who fought SARS (Severe Acute Respiratory Syndrome) and bird flu as Hong Kong’s health director, has warned such cases should not be taken lightly.

“It is true that the number is small, but because of that I would say that we have not seen the full situation or the whole picture of what is happening,” she told El Pais.

No one country, however, is likely to want to be seen as tipping the scale towards a pandemic declaration, especially with discontent rising about whether public health experts have exaggerated the risks of the H1N1 strain.

That leaves the WHO in a difficult spot, given many disease experts anticipate the new virus could rebound with a vengeance later this year when winter temperatures conducive to the spread of flu return to the populated northern hemisphere.

WHO spokesman Gregory Hartl summed up the quandary Monday when asked why international health experts remain on edge about future risks from the flu. “The world really would not forgive us if we did not continue to be vigilant,” he said.

For health authorities worldwide, the question remained how far the virus would spread and how serious would it be. The WHO remained at pandemic alert level 5, meaning a pandemic is imminent.

“If it spreads around the world you will see hundreds of millions of people get infected,” the WHO’s Dr. Keiji Fukuda told a news briefing.

If it continues to spread outside the Americas, the WHO would likely move to phase 6, a full pandemic alert. This would prompt countries to activate pandemic plans, distribute antiviral drugs and antibiotics and perhaps advise people to take other precautions like limiting large gatherings.

“It’s not so much the number of countries, but whether the virus sets up shop in any of those countries like it has here and starts to spread person to person. And given the number of countries that have cases, one would think that eventually that criteria would be met,” said acting CDC director Dr. Richard Besser.

He and Fukuda said it would be important to watch the Southern Hemisphere, where winter and the flu season are just beginning.

Other pandemics have started with a mild new virus in spring that has come back to cause severe disease later in the year. WHO said it would begin sending 2.4 million treatment courses of Roche AG’s and Gilead Sciences Inc’s Tamiflu, an antiviral proven effective against the new flu, to 72 nations.