Pandemic Periods and Phases

PDF Version

An influenza (flu) pandemic can occur when a change takes place in an influenza A virus, causing the emergence of a new strain to which people have little or no immunity. If this new subtype, or strain, has the ability to spread easily from person to person, many people around the world could become ill and possibly die. This is referred to as an influenza pandemic.

The World Health Organization (WHO) has identified three distinct pandemic periods:

1. the interpandemic period, when there are outbreaks of influenza in animals and/or birds but no new influenza strains are detected in humans;
2. the pandemic alert period, characterized by human outbreaks of a new influenza strain; and
3. the pandemic period, with sustained human-to-human transmission of the virus in the general population.

Each period is further subdivided in specific phases, according to the assessed risk of a pandemic. Changes from one phase to another are triggered by several factors, including the spread of the disease among humans and the characteristics of circulating viruses. Each phase coincides with a series of recommended activities to be undertaken by the WHO, the international community, governments and industry.

Interpandemic period

Phase 1: There have been no new influenza virus subtypes detected in humans that would signal the conditions required for a pandemic. Based on past evidence, the influenza viruses detected in animals are considered to be of low risk to humans.

Phase 2: There have been no new influenza virus subtypes detected in humans. However, a circu- lating animal influenza virus subtype poses a substantial risk of human disease. This assessment is based on various factors, such as past history of a similar strain causing serious illness in humans and the extent of the outbreaks in animals.

Pandemic alert period

Phase 3: A new influenza virus subtype is detected in humans. There may be rare instances of an infected individual spreading the virus to other individuals they have been in close contact with, but in general there is no evidence of the virus spreading easily among humans.

Phase 4: Small clusters of human-to-human spread of the virus are reported. But outbreaks are localized, which suggests that the virus does not spread easily to and among humans.

Phase 5: One or more larger clusters are reported, but human-to-human spread is still localized, which suggests that the virus is becoming increasingly capable of infecting humans but may not be fully transmissible (there is a substantial pandemic risk).

Pandemic period

Phase 6: The virus is easily transmitted to and among humans, resulting in increased and sustained spread of the virus in the general population.