WHO's scale of global disease threat

While the number of new swine flu cases has plunged dramatically around the world over the past couple of months, the World Health Organization is not yet ready to say the 2009 flu pandemic is over.

The organizationsays it wants to wait a few weeks to seewhether rising levels of infection in West Africa and the risk posed by the winter months in the southern hemisphere rekindlethe pandemic before declaring that the pandemic has moved out of Phase 6, the highest alert level, and into the post-peak period.

WHO says while pandemic flu activity must drop significantly for the pandemic alert to be downgraded, the organization remains uncertain whether additional waves will occur. WHO warns countries will need to be prepared for another wave, as waves can be separated by months.

The organization won't consider advising global health authorities that the swine flu has moved from Phase 6,into the post-peak period and finally to the post-pandemic period until influenza disease activity returns to levels normally seen for seasonal influenza.

"There is no on and off switch for a pandemic. It's not a single event. What we have to see is that the behaviour of the H1N1 virus becomes like the behaviour of other seasonal viruses," WHO spokesman Gregory Hartl told a news briefing on Feb. 23.

The pandemic's progress

The WHO officially declared a pandemic of H1N1 swine flu on June 11, 2009, marking the first global pandemic in 40 years.

The announcement meant that the swine flu virus was spreading from person to person in a sustained manner outside of North America, where the disease appeared to have originated.

By the June 11 announcement, the outbreak had officially progressed to Phase 6 of the WHO's scale of pandemic alert. Phase 6 is defined as "community level outbreaks in at least one other country in a different WHO region in addition to the criteria defined in Phase 5. Designation of this phase will indicate that a global pandemic is underway."

The criteria defined in Phase 5 include human-to-human spread of the virus into at least two countries in one WHO region.

The WHO declared Phase 5 underway on April 29, 2009, which meant the new strain of H1N1 influenza A virus that first appeared in Mexico was being regularly transmitted between people who hadn't been to Mexico.

Phase 5 indicates:

• Sustained human-to-human transmission of a virus in at least two countries in one region.
• Community-level outbreaks could occur.
• The outbreak is unlikely to fizzle out.

Raising the levels

In raising the pandemic level, the WHO considers three factors:

• Is the virus new?
• Does it cause severe disease?
• Does it move efficiently between people?

Phase 4 is characterized by "verified human-to-human transmission of an animal or human-animal influenza reassortant virus able to cause 'community-level outbreaks.'" While it signifies a major increase in the threat of a pandemic, Phase 4 does not mean a pandemic is inevitable.

In Phases 1-3, the virus normally circulates continuously only in animals. In Phase 2, there may be reports of animal-to-human transmission, making the virus a potential pandemic threat. In Phase 3, there may be small clusters of infection in humans, but not enough human-to-human transmission to cause community-level outbreaks.

Declaring a pandemic signals governments to spend more on containing the virus and drug makers to speed up development of a swine flu vaccine, but does not necessarily mean the virus is causing more severe illnesses or deaths.

Swine flu numbers

While there were deaths, most people who came down with swine flu had mild cases.

Still, WHO estimates that there were 16,000 laboratory confirmed deaths around the world between April 2009 and late February 2010. But the organization cautions that it will be a year or two before an accurate number of deaths can be determined.

Each year, health authorities estimate the number of people killed by seasonal flu using statistical models. Those models haven't yet been developed for determining deaths by swine flu.

Pandemic forecasts estimated that up to 300,000 Canadians might get sick enough with swine flu to require hospitalization. Health Canada reports that as of Feb. 13, 2010 about 8,600 people were admitted to hospital suffering from swine flu. Of them, 1,449 spent some time in intensive care units, and there were 423 confirmed swine flu deaths.

In the first six weeks of 2010, one person died of swine flu and seven people required treatment in hospital. On Jan. 27, 2010, the Public Health Agency of Canada declared that the second wave of swine flu had tapered off.

WHO warns against complacency

The last pandemic the Hong Kong flu of 1968 killed about one million people. Ordinary seasonal flu kills about 250,000 to 500,000 people each year.

The WHO's Dr Keiji Fukuda warned the Council of Europe hearing on pandemic flu on Jan. 26, 2010, not to get complacent about the swine flu pandemic, even thoughthis pandemic has not been nearly as harsh as others that killed millions over the past century.

"The labelling of the pandemic as 'fake' is to ignore recent history and science and to trivialize the deaths of over 16,000 people and the many additional serious illnesses experienced by others."

The organization says the worst may be over but it may also be yet to come. The virus could still mutate and combine with avian flu, or a third, possibly more dangerous wave, could develop.