Bulletin of the World Health Organization

The elusive definition of pandemic influenza

Peter Doshi

Volume 89, Number 7, July 2011, 532-538

Table 1. World Health Organization (WHO) pandemic influenza guidelines, 1999–2009

WHO pandemic influenza guidelines Contains definition of pandemic influenza? Contains clear basis for declaring a pandemic? Content
199917 Unclear (nothing presented as a formal definition) Yes Text most resembling a definition of pandemic influenza: “At unpredictable intervals, however, novel influenza viruses emerge with a key surface antigen (the haemagglutinin) of a totally different sub-type from strains circulating the year before. This phenomenon is called “antigenic shift”. If such viruses have the potential to spread readily from person-to-person, then more widespread and severe epidemics may occur, usually to a similar extent in every country within a few months to a year, resulting in a pandemic” (p. 6)
Basis for declaring a pandemic: “The pandemic will be declared when the new virus sub-type has been shown to cause several outbreaks in at least one country, and to have spread to other countries, with consistent disease patterns indicating that serious morbidity and mortality is likely in at least one segment of the population” (p. 14)
200518 No Yes A pandemic will be said to have begun when a newa influenza virus subtype is declared to have reached Phase 6. Phase 6 is defined as “Increased and sustained transmission in the general population” (p. 9)
200919 No Yes WHO writes, “Phase 6, the pandemic phase, is characterized by community level outbreaks in at least one other country in a different [second] WHO region in addition to the criteria defined in Phase 5. Designation of this phase will indicate that a global pandemic is under way” (p. 26)
Phase 5: “The same identified virus has caused sustained community level outbreaks in at least two countries in one WHO region” (p. 27)
Phase 4: “Human-to-human transmission of an animal or human-animal influenza reassortant virus able to sustain community-level outbreaks has been verified” (p. 27)

a WHO provides a “Definition of new: a subtype that has not circulated in humans for at least several decades and to which the great majority of the human population therefore lacks immunity” (p. 6).