Table 2 World Health Organization phases and UK alert levels.
| Inter‐pandemic 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, 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† and response to additional cases |
| Phase 4: Small cluster(s) with limited human‐to‐human transmission but spread is highly localised, suggesting that the virus is not well adapted to humans† |
| Phase 5: Larger cluster(s) but human‐to‐human spread still localised, suggesting that the virus is becoming 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† |
| UK Alert Level 1: Virus/cases only outside the UK |
| UK Alert Level 2: Virus isolated in the UK |
| UK Alert Level 3: Outbreak(s) in the UK |
| UK Alert Level 4: Widespread activity across the UK |
| Post‐pandemic period |
|---|
| Return to inter‐pandemic arrangements of phase 1 |
*The distinction between phases 1 and 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 localised or widespread, and/or other scientific parameters.
†The distinction between phases 3, 4 and 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.