Facts About the Definition of the Pandemic Influenza (H1N1) 2009 and Vaccine Safety

A pandemic is defined as the emergence of a new influenza virus that has caused sustained community outbreaks in two or more countries in a WHO Region and sustained community outbreaks in at least one other country in a different WHO Region. The criteria for the definition of a pandemic remain geographical dispersion and viral transmission. The clinical severity of the disease is an important consideration but differs from geographical dispersion, and is not currently included in the definition of a pandemic. WHO is working in substantial and measurable ways to incorporate definitions of clinical severity in the general definition of a pandemic.