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World Health Organisation (WHO) International Pandemic Phases

Inter-pandemic period

Phase 1

No new influenza virus subtypes have been detected in humans. An influenza virus subtype that has caused human infection may be present in animals. If present in animals, the risk(1) of human infection or disease 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.

Pandemic alert period

Phase 3

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

Phase 4

Small cluster(s) with limited human to-human transmission but spread is highly localized, suggesting that the virus is not well adapted to humans(2)

Phase 5

Larger cluster(s) but human-to-human spread still localized, suggesting that the virus is becoming increasingly better adapted to humans, but may not yet be fully transmissible (substantial pandemic risk).b

Pandemic period

Phase 6

Pandemic phase: increased and sustained transmission in general population.

Post pandemic Period

Return to inter-pandemic period

(1) The distinction between phase 1 and phase 2 is based on the risk of human infection or disease resulting from circulating strains in animals. The distinction would be based on various factors and their relative importance according to current scientific knowledge. Factors may include: pathogenicity in animals and humans; occurrence in domesticated animals and livestock or only in wildlife; whether the virus is enzootic or epizootic, geographically localized or widespread; other information from the viral genome; and/or other scientific information

(2) The distinction between phase 3, phase 4 and phase 5 is based on an assessment of the risk of a pandemic. Various factors and their relative importance according to current scientific knowledge may be considered. Factors may include: rate of transmission; geographical location and spread; severity of illness; presence of genes from human strains (if derived from an animal strain); other information from the viral genome; and/or other scientific information.

Alert Levels

The World Health Organisation (WHO) has moved to Alert Level 6 – indicating a substantial pandemic risk.
WHO alert phases

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