"The world is moving into the early days of its first influenza pandemic in the 21st century," WHO chief Dr. Margaret Chan told reporters after the U.N. agency held an emergency meeting with flu experts and moved the agency's alert level to phase 6, its highest. "The (swine flu) virus is now unstoppable."
On Thursday, WHO said 74 countries had reported 28,774 cases of swine flu, including 144 deaths. Chan described the virus as "moderate." According to WHO's pandemic criteria, a global outbreak has begun when a new flu virus begins spreading in two world regions.
According to the criteria, the WHO should have declared the pandemic about four weeks ago, said Laurie Garrett, senior fellow for global health at the Council on Foreign Relations.
Hear Garrett discuss the WHO alert system and implications of the announcement:
The WHO has stressed that most cases are mild and require no treatment, but the fear is that a rash of new infections could overwhelm hospitals and health authorities -- especially in poorer countries.
Still, about half of the people who have died from swine flu had been young and healthy -- people who are not usually susceptible to flu. Swine flu is also crowding out regular flu viruses. Both features are typical of pandemic flu viruses.
Chan also pointed out that a pandemic denotes the geographical spread of a disease, not necessarily the severity.
"We also want to make clear that the higher level of the pandemic does not necessarily mean we are going to see a more dangerous virus or see many more people falling severely ill or dying," she said.
Many experts agreed that the pandemic declaration did not mean the virus was getting deadlier.
"People might imagine a virus is now going to rush in and kill everyone," John Oxford, a professor of virology at St. Bart's and Royal London Hospital, told the Associated Press. "That's not going to happen."
But he said the swine flu virus might evolve into a more dangerous strain in the future. "That is always a possibility with influenza viruses," he said. "We have to watch very carefully to see what this virus does."
The long-awaited announcement is scientific confirmation that a new flu virus has emerged and is quickly circling the globe. WHO will now ask drugmakers to speed up production of a swine flu vaccine. The declaration will also prompt governments to devote more money toward efforts to contain the virus.
But the WHO stuck by its recommendation against travel restrictions thus far. "We do not recommend closure of borders," Chan said. "There should not be restriction of movement of people, goods and services. Those are important recommendations, they stand."
The last pandemic -- the Hong Kong flu of 1968 -- killed about 1 million people. Regular flu kills about 250,000 to 500,000 people annually.
Swine flu is also continuing to spread during the start of summer in the northern hemisphere. Normally, flu viruses disappear with warm weather, but swine flu is proving to be resilient.
The pandemic designation might have been made much earlier if WHO had more accurate information about swine flu's rising sweep through Europe. Chan said she called the emergency meeting with flu experts after concerns were raised that some countries like Britain were not accurately reporting their cases.
After Thursday's meeting, Chan said the experts agreed there was wider spread of swine flu than what was being reported.
Chan would not say which country tipped the world into the pandemic, but said all countries and experts were agreed that it was time to declare a global outbreak.
Drug giant GlaxoSmithKline PLC said they could start large-scale production of pandemic vaccine in July but that it would take several months before large quantities would be available.
Many health experts say WHO's pandemic declaration could have come weeks earlier but the agency became bogged down by politics. In May, several countries urged WHO not to declare a pandemic, fearing it would cause social and economic turmoil.
"This is WHO finally catching up with the facts," Michael Osterholm, University of Minnesota flu expert who has advised the U.S. government on pandemic preparations, told the AP.
Despite WHO's hopes, the announcement will almost certainly spark panic about spread of swine flu in some countries. Fear has already gripped Argentina, where thousands of people worried about swine flu flooded into hospitals this week, bringing emergency health services in the capital of Buenos Aires to the brink of collapse. Last month, a bus arriving in Argentina from Chile was stoned by people who thought a passenger had swine flu, according to media reports.
In the United States, where there have been more than 13,000 cases and at least 27 deaths from swine flu, officials at the Centers for Disease Control and Prevention said the move would not change how the U.S. handled H1N1 flu.
"Our actions in the past month have been as if there was a pandemic in this country," Glen Nowak, a CDC spokesman, said Thursday.
The U.S. government has already taken steps like increasing availability of flu-fighting medicines and authorizing $1 billion for the development of a new vaccine against the novel virus. In addition, new cases seem to be declining in many parts of the country, U.S. health officials say, as North America moves out of its traditional winter flu season.
Still, New York City reported three more swine flu deaths Thursday, including one child under the age of 2.