and Environment, WHO said "An easy way to think about pandemic... is to say: a pandemic is a global outbreak. Then you might ask yourself: 'What is a global outbreak?' Global outbreak means that we see both the spread of the agent... and then we see disease activities in addition to the spread of the virus." In planning for a possible influenza pandemic, the WHO published a document on pandemic preparedness guidance in 1999, which was revised in 2005 and 2009. It defined phases and appropriate actions for each phase in an aide-mémoire titled WHO pandemic phase descriptions and main actions by phase. The 2009 revision, including descriptions of a pandemic and the phases leading to its declaration, was finalized in February 2009. The 2009 H1N1 virus pandemic was neither on the horizon at that time nor mentioned in the document. All versions of this document refer to influenza. The phases are defined by the spread of the disease; virulence and mortality are not mentioned in the current WHO definition, although these factors have previously been included. In 2014, the United States Centers for Disease Control and Prevention introduced an analogous framework to the WHO's pandemic stages titled the Pandemic Intervals Framework. It includes two pre-pandemic intervals, Investigation Recognition and four pandemic intervals, Initiation Acceleration Deceleration Preparation It also includes a table defining the intervals and mapping them to the WHO pandemic stages. Severity In 2014, the United States Centers for Disease Control and Prevention adopted the Pandemic Severity Assessment Framework (PSAF) to assess the severity of pandemics. The PSAF superseded the 2007 linear Pandemic Severity Index, which assumed 30% spread and measured case fatality rate (CFR) to assess the severity and evolution of the pandemic. Historically, measures of pandemic severity were based on the case fatality rate. However, the case fatality rate might not be an adequate measure of pandemic severity during a pandemic response because: Deaths may lag several weeks behind cases, making the case fatality rate an underestimate The total number of cases may not be known, making the case fatality rate an overestimate A single case fatality rate for the entire population may obscure the effect on vulnerable sub-populations, such as children, the elderly, those with chronic conditions, and members of certain racial and ethnic minorities Fatalities alone may not account for the full effects of the pandemic, such as absenteeism or demand for healthcare services