Students wearing masks leave their primary school in Hong Kong on Thursday. Hong Kong's government has ordered all kindergartens and primary schools closed for two weeks after a dozen students tested positive for the swine flu. (Kin Cheung/Associated Press)

Swine flu, or H1N1 influenza, has reached the pandemic level, the World Health Organization warned Thursday — marking the first time it has called a global flu epidemic in 41 years.

"The world is moving into the early days of its first influenza pandemic in the 21st century," WHO's director-general, Margaret Chan, announced in Geneva after consulting health experts in an emergency meeting.

"The virus is now unstoppable. However, we do not expect to see a sudden and dramatic jump in the number of severe and fatal infections," she added.

WHO director-general Margaret Chan leaves a news conference at the World Health Organization in Geneva on Thursday. (Anja Niedringhaus/Associated Press)

The announcement means the H1N1 swine flu virus is spreading from person to person in a sustained manner in at least two regions. The swine flu started in North America, but cases have been mounting in Australia, the United Kingdom, Japan and Chile. Further spread is "inevitable," Chan said.

The geographic spread fulfils the UN health organization's criteria for declaring a pandemic and moving its alert to Phase 6 — the highest level.

The pandemic declaration sends a signal to governments to spend more on containing the virus and to drugmakers to speed up the development of a swine flu vaccine. However, it does not mean the virus is causing more severe illnesses or deaths.

Countries like Canada told to brace for 2nd wave

In countries like Canada that already have a sustained spread in communities, Chan urged governments to brace for a second wave of cases.

In Ottawa, Health Minister Leona Aglukkaq said she wanted to reassure Canadians that the government and public health officials "were prepared for this decision by the WHO and this decision does not change our approach here in Canada."

"Canada is well-prepared for the situation. We have a national plan that we have been following."

The declaration acknowledges the reality of the virus's spread, said Dr. Allison McGeer, an influenza expert at Toronto's Mount Sinai Hospital.

"From a science perspective, anybody will tell you that this has been a pandemic for weeks," said McGeer.

"At this early stage, the pandemic can be characterized globally as being moderate in severity," WHO said in the statement, again urging nations not to close borders or restrict travel and trade. "[We] remain in close dialogue with influenza vaccine manufacturers."

Canada's chief public health officer, Dr. David Butler-Jones, said drugmaker GlaxoSmithKline has received a seed strain, or raw ingredients, for a pandemic vaccine from WHO.

The company will use the seed strain to develop a swine flu vaccine, which will be tested for safety in clinical trials. The aim is to have a commercial vaccine ready in the fall, he said.

Dr. Thomas Frieden, director of the U.S. Centers for Disease Control and Prevention, told reporters WHO "waited until they were certain that they had documentation that in multiple continents there was person-to-person sustained transmission."

The declaration marks the first pandemic call since 1968, when Hong Kong flu killed about one million people.

'It is likely in light of sustained community transmission in countries outside of North America — most notably in Australia — that Level 6 will be declared.'— Scotland's Health Secretary Nicola Sturgeon

Before making the declaration, Chan held an emergency conference call with leading flu experts to discuss the outbreak of the virus. It has caused nearly 27,737 cases of swine flu, including 141 deaths.

Seasonal flu kills about 250,000 to 500,000 people each year.

The WHO had been trying to ready the world for a pandemic declaration for some time, saying the new H1N1 virus showed no signs of abating.

Moderate effects

Countries' individual pandemic plans could include investing more money in health services, imposing quarantines, closing schools, travel bans and trade restrictions.

The WHO has said it does not support travel bans or trade restrictions in the wake of swine flu.

"The disease is pretty moderate in its effects so far, so you wouldn't want to disrupt daily life too much," said WHO spokesman Gregory Hartl. "We're not in an Armageddon scenario."

Dr. Anne Schuchat, director of CDC's National Center for Immunization and Respiratory Disease, said it is "really important to remember that a pandemic is a global thing."

"It means that the whole world is united around this condition now."

Drug companies have told WHO that production of seasonal flu vaccination is almost complete and they are ready to start developing pandemic swine flu vaccine.

A limited number of doses could be ready in September, said WHO vaccine expert Marie Paule Kieny. Regulatory agencies need to fast-track registration after ensuring the pandemic vaccine is safe, and countries need to prioritize who has access to it, she added.

Spike in Australian infections

Donations of antiviral medications have been sent to poorer countries, Chan noted.

The spike in infections in Australia is one reason for the heightened pandemic alert level. The WHO listed Australia's confirmed cases at 1,224 by late Wednesday.

Aglukkaq announced Thursday that the federal government has sent two nurses and specialists, including a pediatrician, to help with an outbreak at St. Theresa Point in Manitoba. The community about 500 kilometres from Winnipeg is home to more than 200 people who have fallen ill.

Also on Thursday, Nunavut announced 47 new cases of swine flu in the territory. The federal government is also sending an epidemiologist to the territory to help with the outbreak investigation.

As of Wednesday, 2,978 laboratory-confirmed cases of H1N1 flu virus have been reported in all provinces and territories except Newfoundland and Labrador, according to the Public Health Agency of Canada.

There have been four deaths and 131 hospitalizations related to swine flu in the country.

Dr. Donald Low, medical director of Ontario's public health laboratories, said much of the world has already been treating the virus as though it was a pandemic for several weeks.

"I don't see what would possibly change, calling this a pandemic," Low said.

The virus is already widespread across Canada, and Low doesn't foresee any panic being triggered by a pandemic declaration.

With files from The Associated Press and The Canadian Press