At irregular intervals, usually of a few decades, a new influenza virus emerges which is novel to all or most people which means that there can be little specific immunity among humans except for older people who may have met unless they met the same virus. This new virus can then spread rapidly from human to humans all over the world. Because of the lack of human immunity the virus is often more aggressive and causes more serious disease and deaths.

At least some of the three pandemics during the last (20th) century are thought to have come from an animal or bird influenza virus that itself mutated or swapped genes with a human (so called recombination) and acquired the ability to both infect humans and, more importantly, spread between them. It is thought that as an animal/bird influenza adapts to humans and becomes transmissible, it also loses some of its pathogenicity for humans. This is of evolutionary value as the new strain is more likely to survive if it does not kill its new human host.

Eventually as immunity increases among humans, and the pandemic virus changes, the pandemic strain becomes part of (and tends to dominates) the mix of seasonal influenza viruses.

Related documents

  Review of ECDC’s response to the influenza pandemic 2009/10 (November 2011)

 Technical report: Pandemic influenza preparedness in the EU/EEA 2007 (Annex 1, Annex 2,  Annex 3)

 Report for policy makers: Pandemic Preparedness in the European Union 2007

 Influenza Surveillance in a Pandemic (Paper from ECDC working group, August 2007)

 Special Report:  Innovations in EU pandemic preparedness, September 2007

 Technical report: Pandemic influenza preparedness in the EU 2006 

  Third joint EC/ECDC/WHO Workshop on Pandemic Influenza Preparedness (May 2006)