GENEVA -- A(H1N1) flu is now formally a pandemic, a declaration by UN health officials that will speed vaccine production and spur government spending to combat the first global flu epidemic in 41 years.

Thursday's announcement by the World Health Organization doesn't mean the virus is any more lethal, only that its spread is considered unstoppable.

Cleaners wear face masks at Saint Gabriel School in Bangkok June 12, 2009. [Agencies]

Since it was first detected in late April in Mexico and the United States, A(H1N1) flu has reached 74 countries, infecting nearly 29,000 people. Most who catch the bug have only mild symptoms and don't need medical treatment.

"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 US 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, US health officials have been treating it as a pandemic, increasing the availability of anti-viral 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 may change into a more frightening form in the near future, and so people should not be complacent, he added.

So far, A(H1N1) flu has caused 144 deaths, compared with ordinary flu that kills up to 500,000 people a year.

The pandemic decision might have been made much earlier if WHO had more accurate information about A(H1N1) 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.

Chan said the experts unanimously agreed there was a wider spread of A(H1N1) flu than was being reported.

She would not say which country tipped the world into the pandemic, but WHO flu chief Keiji Fukuda said the situation from Australia seemed to indicate the virus was spreading rapidly there, more than 1,300 cases were reported by Thursday.

In Chile, authorities have identified almost 1,700 cases to WHO.

Many health experts said the world has been in a pandemic for weeks but WHO became too bogged down by politics to declare one. In May, several countries urged WHO not to declare a pandemic, fearing it would cause social and economic turmoil. At the time, WHO said it would rewrite its pandemic definition to avoid announcing one.

But with the recent surge in cases across Europe, Chile, Australia and Japan, the agency was under increasing pressure to acknowledge a pandemic.

"This is WHO finally catching up with the facts," said Michael Osterholm, a flu expert at the University of Minnesota.

David Ropeik, an expert in risk perception and communication at Harvard University, says the word pandemic is less frightening than when emerged during worries about bird flu a few years ago.

He said the "soft buildup" to declaring A(H1N1) flu a pandemic has been helpful.

"That allows people to get used to what is otherwise a scary word, understand the particulars of the disease, and that should mean reaction will be a little more information-based and a little less emotional," Ropeik said in an e-mail.