The World Health Organization has declared that the H1N1 pandemic, which killed more than 18,000 people around the world and sparked mass vaccination programs, is over.

WHO director general Margaret Chan said Tuesday that the pandemic has "largely run its course." 

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Margaret Chan, director general of the World Health Organization, has declared the H1N1 pandemic over. ((Keystone, Laurent Gillieron/AP))

"As we enter the post-pandemic period, this does not mean that the H1N1 virus has gone away," Chan told reporters in a telephone briefing from her native Hong Kong.

"Based on 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."

The WHO and its international group of influenza experts has been watching for a number of signs it believes are key to determining that H1N1 is morphing into a seasonal flu virus that circulates during winter months in the northern and southern hemispheres.

During the pandemic, the H1N1 virus spread widely out of season, crowding out other flu viruses, Chan said, but this is no longer the case.

Stockpiled H1N1 flu vaccines are still effective against the strain and can be given to high-risk groups, the WHO said. 

The WHO says the virus has killed at least 18,500 people worldwide since it emerged in the spring of 2009. 

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The H1N1 vaccine proved to be a good match, WHO said.

On Jan. 27, the Public Health Agency of Canada announced that the second wave of pandemic H1N1 2009 had tapered off. As of then, a total of 426 H1N1 deaths were reported to the agency since the beginning of the pandemic.

Seasonal flu kills 4,000 to 8,000 Canadians every year, mostly elderly individuals, federal health officials estimate.

Pandemic call questioned

WHO has been criticized for calling H1N1 a pandemic when the outbreak turned out to be milder than feared.

Some public health officials supported the pandemic call, saying H1N1 caused serious illnesses in younger groups that flu traditionally doesn't harm seriously. 

Chan has rejected allegations that WHO allowed influence from drug companies to affect its decision to declare the pandemic.

She acknowledged that WHO will review its pandemic phases to take severity of a virus into account.

"This time around, we have been aided by pure good luck. The virus did not mutate during the pandemic to a more lethal form. Widespread resistance to [the antiviral] oseltamivir did not develop. The vaccine proved to be a good match with circulating viruses and showed an excellent safety profile," Chan said.

Dry run

Extensive preparations and support from the international community, including countries with weak health systems, helped to detect cases and report them promptly, she said.

"Had things gone wrong in any of these areas, we would be in a very different situation today." 

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There were long lines at H1N1 immunization centres last October.

H1N1 amounted to a practice run for health officials, said Dr. Allison McGeer, director of infection control at Mount Sinai Hospital in Toronto.

In Canada, most provinces are reviewing their pandemic plans. About 40 per cent of Canadians were immunized against H1N1, a key issue for health officials who blame uneven and unpredictable distribution of the vaccine.

"I think most of us in the flu business would have liked a higher vaccination rate," McGeer said. "A high vaccination rate in Year 1 would offer us more substantial protection this year." 

In the early part of the pandemic, McGeer added that getting antiviral treatment out to some high-risk groups also could have been improved.

Chan warned countries not to become complacent and to keep watch for any unusual patterns of infection or mutation that could reduce the effectiveness of current vaccines and antiviral medications to flu viruses — including the H5N1 bird flu virus that has infected 503 people over seven years, killing 299.

WHO's flu chief, Keiji Fukuda, said the true death toll from H1N1 is likely higher but won't be known for months.

With files from The Associated Press and The Canadian Press