Interpandemic 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 of human infection or disease is considered to be low.
Strengthen influenza pandemic preparedness at the global, regional, national and subnational levels.

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.
Minimize the risk of transmission to humans; detect and report such transmission rapidly if it occurs.

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.
Ensure rapid characterization of the new virus subtype and early detection, notification and response to additional cases.

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.
Contain the new virus within limited foci or delay spread to gain time to implement preparedness measures, including vaccine development.

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).
Maximize efforts to contain or delay spread, to possibly avert a pandemic, and to gain time to implement pandemic response measures.

Pandemic period

Phase 6
Pandemic: increased and sustained transmission in general population.
Minimize the impact of the pandemic.

From the World Health Organization