Factbox: The WHO's pandemic alert phases

(Reuters) - The World Health Organization will announce on Tuesday whether the H1N1 flu outbreak has been downgraded from a pandemic status.

The WHO uses a series of six phases of pandemic alert to inform the world about an outbreak and the need to launch progressively more intense preparations and measures.

The phases at the upper end of the scale mark the geographic spread of the circulating influenza, not its severity.

Margaret Chan, Director-General of the United Nations agency, is charged with deciding on when to move from one phase to another. Her decision is based on advice from international public health experts.

Those experts, meeting in the WHO's 15-member emergency committee, convened by teleconference on Tuesday to assess whether evidence from the southern hemisphere winter suggests the H1N1 outbreak is now subsiding.

The current phase of alert is 6 on a scale of 1 to 6 -- a full-blown pandemic that requires sustained, human-to-human spread over many countries of a new and serious virus.

The experts can recommend that Chan should declare the pandemic is over ("post pandemic"), has subsided but could flare up again ("post peak") or that further evidence is needed and the pandemic is still underway.

Here are details of the six phases:

-- PREDOMINANTLY ANIMAL INFECTIONS; FEW HUMAN INFECTIONS:

* PHASE 1:

No reports of human infections from influenza viruses circulating naturally among animals and birds.

* PHASE 2:

Some infections in humans from influenza viruses in animals show the virus is a potential pandemic threat.

* PHASE 3:

Sporadic cases or small clusters of infection in people, but not enough to result in sustained human-to-human transmission.

-- SUSTAINED HUMAN-TO-HUMAN TRANSMISSION

* PHASE 4:

The virus is spreading from person to person causing outbreaks at the level of communities, indicating an increased risk but not certainty that it will turn into a pandemic.

-- WIDESPREAD HUMAN INFECTION/PANDEMIC

* PHASE 5:

Human-to-human spread of the virus in at least two countries in one region. Most countries are not yet affected but a pandemic is imminent. Health authorities have little time left to finalize measures to deal with the outbreak.

* PHASE 6:

The virus is spreading in at least two regions, indicating a global pandemic is underway.

-- POSSIBILITY OF RECURRENT EVENTS

* POST PEAK

Pandemic activity is decreasing in most countries but could flare up again and governments must be prepared for a second wave, maybe months after the initial pandemic.

-- DISEASE ACTIVITY AT SEASONAL LEVELS

* POST PANDEMIC

The virus is now behaving like other forms of seasonal influenza.

Source: World Health Organization (WHO pandemic phase chart at link.reuters.com/gyz24n )

(Compiled by Jonathan Lynn; editing by David Stamp)