The worldwide swine-flu pandemic is officially over, the World Health Organization announced Tuesday, more than a year after the outbreak that killed more than 18,000 people began.

The agency's committee of flu experts told WHO Director-General Margaret Chan the pandemic had "largely run its course" and the world is no longer at phase six, the highest influenza alert level.

Chan said the virus has entered the "post-pandemic phase," as its presence around the world has diminished to levels akin to those for seasonal influenza.

Despite the positive news, Chan warned health officials around the world to remain vigilant for spikes in infection rates or disease that is resistant to treatment, which could suggest the virus is mutating, rendering current vaccines useless.

"It is likely that the virus will continue to cause serious disease in younger age groups," Chan told reporters during a media briefing from Hong Kong. Chan said high-risk groups, including pregnant women, should continue to get vaccinated against the virus.

"Based on the experience with past pandemics, we expect the H1N1 virus to take on the behaviour of a seasonal influenza virus and continue to circulate for some years to come," Chan said.

She also suggested that health officials are concerned about a potential pandemic that could be even deadlier than swine flu turned out to be.

"Lurking in the background we still have H5N1," she said, in reference to a strain of bird flu.

Since the swine flu outbreak began in April 2009, the virus has killed 18,449 people, including 426 in Canada. More than 8,600 people were hospitalized in this country due to the virus.

Despite fears the H1N1 virus might kill hundreds of thousands more, the virus petered out and many countries have since ended their vaccination and hand-washing campaigns.

The Canadian government had ordered more than 50 million doses of H1N1 vaccine, leaving it with a massive surplus. But Health Minister Leona Aglukkaq on Tuesday credited the government's vaccination campaign with keeping Canada's infection and death rates low, despite the fact less than half of Canadians got vaccinated.

"This country is a global model for its safe and effective rollout of a pandemic vaccine campaign which resulted in a successful rate of about 45 per cent of Canadians who rolled up their sleeves," media relations officer Jenny Van Alstyne said in an email.

"This high vaccination rate played a critical role in the prevention and spread of the virus."

Fears over a massive worldwide death toll were eventually replaced with criticism of the WHO, which some said overstated the virus's threat.

Chan defended the agency's decision to label the swine flu outbreak a pandemic, given the information it had about the virus and its system for determining pandemics.

"This pandemic has turned out to be much more fortunate than what we feared a year ago," Chan said. "This time around we were able through pure good luck that the virus did not mutate to a more lethal form."

However, Chan did say the agency will review its phases for pandemics.

With files from The Associated Press and The Canadian Press