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Table 1 WHO pandemic phases

From: Threat of an influenza pandemic: family physicians in the front line

  Pandemic phases Public health goals
  Inter-pandemic period
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 sub-national levels.
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
3 Human infection(s) with a new subtype, but no 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.
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.
5 Large cluster(s) but human to human spread still localized, suggesting that the new 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
6 Pandemic: increased and sustained transmission in general population. Minimize the impact of the pandemic.
  Post-pandemic period
  Post-pandemic period: return to inter-pandemic period. Return to inter-pandemic period.