The Phases of a Pandemic
The World Health Organization (WHO) has published its newly revised global influenza preparedness plan. The plan outlines the phases of a pandemic and specifies the role of the WHO, identifies public health goals and makes recommendations for national measures before and during a pandemic. The newly defined pandemic phases and the underlying rationales are being described in detail in Chapter 2 of the WHO’s Plan. Chapter 3 focuses on the health objectives and recommended actions. See also this Wikipedia entry.
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.
- Phase 2
- No new influenza virus subtypes have been detected in humans. However, a circulating animal influenza virus subtype poses a substantial risk1 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).
Pandemic period
- Phase 6
- Pandemic: increased and sustained transmission in general population.
Postpandemic period
Return to interpandemic period.
1 The distinction between phase 1 and phase 2 is based on the risk of human infection or disease resulting from circulating strains in animals. The distinction is based on various factors and their relative importance according to current scientific knowledge. Factors may include pathogenicity in animals and humans, occurrence in domesticated animals and livestock or only in wildlife, whether the virus is enzootic or epizootic, geographically localized or widespread, and/or other scientific parameters. (↑)
2 The distinction between phase 3, phase 4 and phase 5 is based on an assessment of the risk of a pandemic. Various factors and their relative importance according to current scientific knowledge may be considered. Factors may include rate of transmission, geographical location and spread, severity of illness, presence of genes from human strains (if derived from an animal strain), and/or other scientific parameters. (↑)