On 11 June 2009, an influenza pandemic caused by the influenza A (H1N1) virus was declared.
An influenza pandemic occurs when a new influenza virus subtype emerges and spreads easily among human beings.
Pandemics have been observed for several hundreds years. The best documented pandemics occurred during the 20th century, in 1918 (Spanish flu), 1957 (Asian flu) and 1968 (Hong Kong flu). These varied in severity with an estimated 1-50 million excess deaths during the pandemic.
WHO uses a series of six phases of pandemic alert to inform the world of the seriousness of the threat and the need to launch progressively more intense preparedness activities.
Based on assessment of all available information and following several expert consultations on influenza A(H1N1), on 11 June 2009, Dr Margaret Chan, WHO's Director-General, raised the current level of influenza pandemic alert from phase 5 to 6 and thereby declaring an influenza pandemic.
Pandemic Preparedness Phase 6 is characterized by a new virus causing sustained community level outbreaks in at least two WHO regions.
The pandemic phase does not indicate the severity of disease caused by the new virus. The severity of the pandemic is categorized as moderated, aggravated or severe. The influenza A (H1N1) has been classified as a pandemic of moderate severity. WHO will continue to monitor the severity of the pandemic as it may change over time.
