On Wednesday, WHO chief Dr. Margaret Chan quizzed eight countries with large swine flu outbreaks to see if a pandemic, or global epidemic, should be declared. After Chan's teleconference, the agency announced that an emergency meeting with its flu experts would be held Thursday.
Since swine flu first emerged in Mexico and the United States in April, it has spread to 74 countries around the globe. On Wednesday, WHO reported 27,737 cases including 141 deaths. Most cases are mild and require no treatment.
The world is in phase 5 of WHO's pandemic alert scale, meaning a global outbreak is imminent. Moving to phase 6, the highest level, means a pandemic has begun. If that declaration is made, it will push drugmakers to fast-track production of a swine flu vaccine.
Chan says she personally believes that a pandemic is under way, but was seeking clear proof that swine flu is spreading rapidly from person to person outside the Americas before declaring a global epidemic.
"Once I get indisputable evidence I will make the announcement," she told reporters Tuesday.
It would be the first flu pandemic in 41 years, since the Hong Kong flu of 1968.
At GlaxoSmithKline PLC, spokesman Stephen Rea said the company was already working with a key ingredient of the swine flu vaccine to see how quickly doses could be produced. Other major pharmaceuticals like Sanofi Pasteur have also been working on a swine flu vaccine since WHO gave them a "seed stock" of the virus last month created by the U.S. Centers for Disease Control and Prevention.
Rea said it could take up to six months before large amounts of a swine flu vaccine are available. At the moment, GlaxoSmithKline is still making regular flu vaccine, which it expects to be completed by July.
After that, Rea said GlaxoSMithKline could speed up its swine flu vaccine production if a pandemic is declared. Once that announcement is made, the company would be obliged to fulfill contracts it signed with countries including Britain, Belgium and France, promising to provide a pandemic vaccine as soon as possible.
If a global outbreak is announced, countries would likely activate their own pandemic preparedness plans if they haven't already. That could mean devoting more money to health services or imposing measures like quarantines, school closures, travel bans and trade restrictions — some of which WHO opposes.
According to WHO's own pandemic criteria, a global outbreak means a new flu virus is spreading in at least two world regions.
With thousands of cases in North America and hundreds in Japan, Australia and Europe, many experts say that threshold has already been reached, but the U.N. agency has held off on making the pandemic call for political reasons.
"If you look at the science, we were at phase 6 weeks ago," said Michael Osterholm, a flu expert at the University of Minnesota who has advised the U.S. government on pandemic preparations.
"What's happening right now is not about public health surveillance and science," he told The Associated Press in an interview Wednesday. "It's about politics and risk communication."
Osterholm said WHO's delayed decision has cost the agency credibility.
"As soon as you try to incorporate risk messaging into science, you are on a slippery slope," he said. "WHO has exacerbated the issue by dancing around it."
In May, several countries urged WHO not to declare a pandemic, fearing it would spark mass panic. The agency appeared to cave into the requests, saying it would rewrite its definition of a global outbreak so that it wouldn't have to declare one right away for swine flu.
But WHO officials have been concerned in recent days after seeing media reports and health experts discussing more swine flu cases than were being reported by the countries themselves.
In one of the most glaring examples, Britain rushed to report another 75 infections Wednesday, for a total of 750 cases, after some outside health officials said the country was not looking very hard for swine flu.
Britain's Health Protection Agency insists the virus is not spreading in communities. But three Greek students recently returned home after catching swine flu in the U.K., proof the virus is spreading more widely than British authorities admit. Two of the cases were documented in the journal of the European Centre for Disease Prevention and Control.
WHO said announcing a pandemic would not mean the situation was worsening, since no mutations have been detected in the virus to show it is getting more deadly.
One flu expert said WHO's pandemic declaration would mean little in terms of how countries are responding to the outbreak.
"The writing has been on the wall for weeks," said Chris Smith, a flu virologist at Cambridge University, adding he didn't know why WHO had waited so long to declare a pandemic. "WHO probably doesn't want people to panic, but the virus is now unstoppable."