Swine flu is now formally a pandemic, a declaration by U.N. health officials that will speed vaccine production and spur government spending to combat the first global flu epidemic in 41 years.

Yesterday's announcement by the World Health Organization does not mean the virus is any more lethal—only that its spread is considered unstoppable.

Since it was first detected in late April in Mexico and the United States, swine flu has reached 74 countries, infecting nearly 29,000 people. Most who catch the bug have only mild symptoms and do not need medical treatment.

WHO chief Dr. Margaret Chan made the long-awaited declaration after the U.N. agency held an emergency meeting with flu experts, and said she was moving to phase 6—the agency's highest alert level—which means a pandemic is under way.

“;The world is moving into the early days of its first influenza pandemic in the 21st century,”; Chan said in Geneva.

Dr. Thomas Frieden, the new head of the U.S. Centers for Disease Control and Prevention, said in Atlanta that he does not expect widespread public anxiety in the United States as a result of the declaration, noting it came nearly two months after the virus was identified.

For many weeks U.S. health officials have been treating it as a pandemic, increasing the availability of antiviral flu medicines and pouring money into a possible vaccination program. And scientists have grown to understand that the virus is generally not much more severe than the seasonal flu.

“;That helps to tamp down any fears that may be excessive,”; Frieden said at a news conference—his first as CDC director.

But the virus can still be deadly and might change into a more frightening form in the near future, and so people should not be complacent, he added.

So far, swine flu has caused 144 deaths, compared with ordinary flu, which kills up to 500,000 people a year.

The pandemic decision 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, such as Britain, were not accurately reporting their cases.