Pandemic alert phases

The World Health Organization (WHO) has developed an alert system to help inform the world about the development of the different stages towards a pandemic. The alert system is comprised of six phases.

The current situation with the HPAI-H5N1 virus in animals and humans places the world in pandemic alert phase 3. This means that H5N1 is currently causing disease (and deaths) in humans, but is not yet spreading efficiently from human to human. The situation with the ‘Mexican flu’ H1N1 virus placed the world in pandemic alert phase 6 since June 2009. This means that there are large clusters of infection with extensive and sustained human-to-human spread in the general population at different continents.

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, 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.

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. Recently it has been recognized that a very crucial event in the emergence of a pandemic virus is the transition from phase 3 to phase 4, which may go much faster than previously supposed. This is an important insight, since, at this transitional stage, major intervention strategies can still be initiated and implemented successfully in countries with a well-developed pandemic preparedness plan. This proved to be the case with the emergence of the ‘Mexican flu’ pandemic in 2009. It took three days between phase 4 and 5, and six weeks between phase 5 and 6.