Period

Phase

Description

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^a 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^b.

Post pandemic period

Return to inter-pandemic period.