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On 11 June 2009, Dr Margaret Chan, WHO’s Director-General, raised the level of influenza pandemic alert to phase 6, thereby declaring Pandemic (H1N1) 2009. For more information about Pandemic (H1N1) 2009, see the menu item at left.

An influenza pandemic occurs when a new influenza virus subtype emerges and spreads easily among human beings.

Pandemics have been observed for several hundreds of years. The best-documented pandemics occurred during the 20th century, in 1918 (Spanish influenza), 1957 (Asian influenza) and 1968 (Hong Kong influenza). These varied in severity, with an estimated 1-50 million excess deaths during the pandemic.

Pandemic phases

WHO uses a six-phase scale of pandemic alert to inform the world of the global spread of a new virus. The scale is also used as a global framework for countries during pandemic preparedness and response planning.

The pandemic phase does not indicate the severity of disease caused by the new virus. The severity of the pandemic is classified as moderated, aggravated or severe. The current pandemic has been classified as being of moderate severity. WHO will continue to monitor the severity of the pandemic, as it may change over time.