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.
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
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
Phase 6: Pandemic: increased and sustained transmission in general population.