WHO: Swine Flu Now A Pandemic, 1st In 41 Years

For many weeks, U.S. 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, swine 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 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.

Chan said the experts unanimously agreed there was a wider spread of swine flu than was being reported.