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The World Health Organization (WHO) on Monday raised the pandemic swine flu alert level from phase 3 to 4, two levels below the declaration of a full pandemic. The elevated alert means there has been sustained human-to-human transmission of the new A/H1N1 swine flu virus and that scientists now believe government efforts should focus on slowing the spread of the virus rather than containing it at its source.

"We have taken a step in that direction, but a pandemic is not considered inevitable," said Keiji Fukuda, the WHO's interim director-general for health, safety and environment. "The situation is fluid and continues to evolve." (See pictures of the swine flu in Mexico.)

That fluidity was perhaps the only certainty of the fast-changing situation. On Monday morning, a 5.6 earthquake in Mexico added to that country's woes, where the death count from suspected swine flu cases had climbed to 152 and more than 1,600 people across the nation had fallen ill. So far, laboratories have confirmed only 26 of Mexico's cases, including seven deaths, as swine flu. In an effort to stem further spread of the apparently deadly disease, the Mexican government announced that all schools would be closed until at least May 6.

New swine flu cases were also confirmed in Canada, Scotland, Spain and the U.S., where the number of confirmed cases more than doubled to 43 — 28 of them students at the same New York City high school. (All the U.S. patients have thus far recovered with few complications.) The deteriorating situation prompted the U.S. State Department to recommend that travelers postpone "nonessential travel" to Mexico — even after U.S. officials criticized the European Union for issuing a travel ban to the U.S. and Mexico. (See five things you need to know about swine flu.)

"We continue to approach this investigation and our control efforts aggressively," said Richard Besser, acting director for the Centers for Disease Control and Prevention (CDC). "You don't know going into an outbreak what it will look like in the end, and we want to be aggressive."

The WHO also announced that it would begin the process of preparing a swine flu vaccine strain. But the agency recommended that drug companies continue to make a seasonal flu vaccine instead of switching immediately to the production of an A/H1N1 flu vaccine. That's important because drugmakers are currently producing a flu vaccine for the southern hemisphere, where the flu season has just begun. (The flu season in the north has just ended.) If vaccine manufacturing capacity were switched from the standard flu to swine flu, that could create a shortfall in normal flu vaccine, potentially leading to needless deaths should the A/H1N1 swine flu end up petering out on its own. Still, preparations for a swine flu vaccine are being initiated, as any vaccine would take months to produce — a costly delay if the new virus ends up becoming virulent. (Read "CDC Readies Swine Flu Vaccine.")

At the same time, health officials have been quick to stave off unnecessary panic. "This is obviously a cause for concern," said President Barack Obama in a speech to the National Academy of Sciences on Monday morning. "But it's not a cause for alarm." That message was echoed by Secretary of Homeland Security Janet Napolitano, the lead federal official on swine flu, as well as WHO officials and just about every other official connected to the global flu response who spoke to the public on Monday.

Indeed, so far there is very little reason for anyone outside Mexico to be worried about their health. There are relatively few cases in the U.S. and other infected countries outside Mexico, and none of those cases have been serious. The virus appears to be vulnerable to antiviral drugs like Tamiflu, and thanks to global pandemic preparations since the SARS epidemic of 2003 and last year's flu outbreak in Hong Kong, the U.S. and other developed countries maintain large stockpiles of the drug. "We are seeing a much more clear and cogent response than in the past," said Laurie Garrett, senior fellow for global health at the Council on Foreign Relations.

Despite the fact that cases outside Mexico haven't been serious, the situation is far from secure. For one thing, scientists still don't know why the virus appears to have caused more serious disease in Mexico. It could be that the virus has simply been there longer or that patients were not treated quickly enough with antivirals; or it could be that a more serious epidemic is still to come in other parts of the world.

The WHO's decision to raise the pandemic alert level to phase 4 cannot be taken lightly. Although the move will not have much effect on the U.S. response, it will obligate countries that have not yet been infected to step up precautions. For poor nations, that undertaking could be expensive, and may divert resources from other health threats. "The [WHO] was mindful of the fact that a phase change would have social and political implications for everyone," said Fukuda. "But we focused on what we knew about the epidemiology."

What is puzzling, however, is the WHO's decision to escalate the alert now, when the world has most likely missed its chance to contain the virus. When the WHO's pandemic alert system was first conceived, phase 4 was intended to indicate the moment when a new flu virus had been identified and could spread effectively from person to person (as Asia's H5N1's bird flu virus, which reached phase 3, has never been able to do), but was still limited enough that health officials could launch a global effort to contain it and snuff it out with antiviral drugs.

But it's clearly too late for that now — the swine flu virus has jumped across borders, and both the WHO and CDC have acknowledged that containment is no longer an option. So, while raising the alert level, the WHO also recommended that countries do not close borders or impose travel bans. "Restricting travel would have very little effect on stopping the movement of this virus," said Fukuda. At this point, trying to close borders would be like closing the barn door after the horse has bolted — better to focus on community-level protections like better disease surveillance and hygiene. (Read "Battling Swine Flu: The Lessons from SARS.")

For now, there are more questions than answers. Most important among them: Exactly what is going on in Mexico, where epidemiologists are still working to understand the swine flu outbreak? Uncertainty, however, is unavoidable when it comes to influenza — a shifty, erratic virus that is harder to get a handle on than, well, a greased pig. "There is no standard picture for how this will develop," said Fukuda. "We don't know."

Read "How to Deal with Swine Flu: Heeding the Mistakes of 1976."

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• Bryan Walsh
• Global health officials have moved the swine flu alert a step closer to a pandemic, but many questions remain about the prognosis of the outbreak