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Pandemic Stages

Pandemic stages include six phases during three periods, as defined by the World Health Organization. The three periods that make up these stages include the interpandemic period, the pandemic alert period, and the pandemic period. The phases of pandemic stages are categorized according to whether virus subtypes are present in humans, whether the spread is localized or widespread, and other factors.

Pandemic Stages: An Overview

The World Health Organization (WHO) has defined pandemic stages consisting of six phases during three periods. The three periods include:
  • Interpandemic period
  • Pandemic alert period
  • Pandemic period.

Pandemic Stages: Interpandemic Period

The interpandemic period is made up of two phases:
  • Phase 1: In phase 1, there are no new influenza virus subtypes that have been detected in humans. However, 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 low.


  • Phase 2: In phase 2, there are no new influenza virus subtypes that have been detected in humans. However, a circulating animal influenza virus subtype poses a substantial risk of human disease.

Pandemic Stages: Pandemic Alert Period

The pandemic alert period is made up of three phases:
  • Phase 3: In phase 3, there is a human infection(s) with a new subtype but no human-to-human spread or, at most, rare instances of spread through a close contact


  • Phase 4: In phase 4, there is a small cluster(s) with limited human-to-human transmission but spread is highly localized, suggesting that the virus is not well adapted to humans


  • Phase 5: In phase 5, there are larger clusters, but human-to-human spread is still localized, suggesting that the virus is becoming increasingly better adapted to humans but may not yet be fully transmissible (substantial pandemic risk).
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