Phases of a Pandemic
The World Health Organization (WHO) has developed a global influenza preparedness plan. The plan from WHO, which defines the stages of a pandemic, outlines the role of WHO, and makes recommendations for national measures before and during a pandemic. There are four periods and 6 phases to the plan developed by WHO and they are:
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 risk of human disease.
Pandemic alert period
Phase 3 : Human infection(s) with a new subtype exist. During Phase 3 there is no human-to-human spread or at most rare instances of spread to humans during close contact.
Phase 4 : Small cluster(s) of infection have been detected with limited human-to-human transmission. The spread is highly localized suggesting that the virus is not well adapted to humans at this point.
Phase 5 : Larger cluster(s) of infection are present. The spread from human-to-human is still localized, suggesting that the virus is becoming increasingly better adapted to humans but may not yet be fully transmissible. Phase 5 indicates that there is a substantial pandemic risk.
Pandemic Period
Phase 6 : Pandemic: Phase 6 indicates a pandemic due to increased and sustained transmission of the virus in the general population.
Post-pandemic period
Once the virus has run its course and has no further hosts to infect, it will stop spreading. After the Pandemic period there will be a return to the Interpandemic Period (Phase 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 (the ability of a pathogen to produce an infectious disease in an organism) in animals and humans, occurrence in domesticated animals and livestock or only in wildlife, whether the virus is enzootic (the non-human equivalent of endemic and means, in a broad sense, “belonging” or “native to”, “characteristic of”, or “prevalent in” a particular geography, race, field, area, or environment) or epizootic (a disease that appears as new cases in a given animal population, during a given period, at a rate that substantially exceeds what is “expected” based on recent experience), geographically localized or widespread, and other scientific parameters.
The distinction among phases 3 and 4 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 other scientific parameters.
