Avian Influenza and the Pandemic Threat

WHO has currently identified six specific phases that would cover the generation of a pandemic:

Phase 1:

No new influenza subtypes have been detected in humans. An influenza virus subtype that has caused human infection may be present in animals. The risk of human infection is considered to be low.

Phase 2:

No new influenza virus subtypes have been detected in humans. However, a circulating animal influenza virus subtype poses a substantial risk of human disease.

(Phase one and two are described as the inter-pandemic period.)

Phase 3:

Human infection(s) with a new subtype are reported. There are no instances of human to human spread, or at most, rare instances of spread to a close contact.

Phase 4:

Small cluster(s), meaning less than 25 people, lasting less than two weeks, with limited human to human transmission occur, but spread is still highly localized, suggesting that the virus is not well adapted to humans.

Phase 5:

Larger cluster(s), meaning between 25-50 people, lasting from two to four weeks, appear. While human to human transmission is still localized, the virus appears

to be increasingly better adapted to humans. Though it is not yet fully transmissible, there is a substantial pandemic risk.

(Phases 3-5 are described as the pandemic alert period).

Phase 6:

Virus transmission increases significantly, and there is sustained transmissibility in the general population.

Revising the pandemic level (up or down) requires that WHO consult a board of external experts to review all available data. The board will then make recommendations to the WHO Director-General, who will then decide whether the pandemic level should be changed.(Phase 6 is the pandemic period).

Related links

• WHO