WHO will now recommend that pharmaceutical companies make A(H1N1) flu vaccine. The agency typically recommends which flu strains drug companies should use in the vaccines. In a global outbreak, WHO also advises whether companies should make pandemic vaccine.

The decision to make pandemic vaccine is a gamble. Most flu vaccine makers cannot make both regular seasonal flu vaccine and pandemic vaccine at the same time. That means they must decide which one the world will need more.

Japanese elementary school students wear masks as a precaution against the Influenza A (H1N1) virus during a tour of Parliament in Tokyo June 12, 2009. [Agencies]

Drug giant GlaxoSmithKline PLC said it could start commercial production of pandemic vaccine in July but that it would take months before large quantities are available.

Glaxo spokesman Stephen Rea said the company's first doses of vaccine would be reserved for countries who had ordered it in advance, including Belgium, Britain and France. He said Glaxo would also donate 50 million doses to WHO for poor countries.

WHO described the pandemic as "moderate." Fukuda said people should not get overly anxious about the virus. "Understand it, put it in context, and then you get on with things," he said.

Still, about half of the people who have died from A(H1N1) flu were previously young and healthy, people who are not usually susceptible to flu. A(H1N1) flu is also crowding out regular flu viruses. Both features are typical of pandemic flu viruses.

A(H1N1) flu is also continuing to spread during the start of summer in the northern hemisphere. Normally, flu viruses disappear with warm weather, but A(H1N1) flu is proving to be resilient.

"What this declaration does do is remind the world that flu viruses like H1N1 need to be taken seriously," said US Secretary of Health and Human Services Kathleen Sebelius, warning that more cases could crop up in the fall.

Now that a pandemic has been declared, some countries might be prompted to devote more money to containing the virus. Many developed countries have pandemic preparedness plans that link spending to a WHO declaration.

The UN is keen to avoid panic. "We must guard against rash and discriminatory action, such as travel bans or trade restrictions," said UN Secretary-General Ban Ki-moon.

Fear has already gripped Argentina, where thousands have flooded hospitals this week, bringing emergency health services in Buenos Aires to the brink of collapse during winter weather. Last month, a bus arriving in Argentina from Chile was stoned by people who thought a passenger had A(H1N1) flu.

The US government has already increased the availability of flu-fighting medicines and authorized $1 billion for developing a new A(H1N1) flu vaccine. In addition, new cases seem to be declining in many parts of the country, US health officials say, as North America moves out of its traditional winter flu season.