Assessment

Stagesedit

The World Health Organization (WHO) previously applied a six-stage classification to describe the process by which a novel influenza virus moves from the first few infections in humans through to a pandemic. It starts when mostly animals are infected with a virus and a few cases where animals infect people, then moves to the stage where the virus begins to be transmitted directly between people and ends with the stage when infections in humans from the virus have spread worldwide. In February 2020, a WHO spokesperson clarified that "there is no official category for a pandemic".a

World Health Organization (WHO) influenza pandemic phase descriptions

Phase 1	Phase 2	Phase 3	Phase 4	Phase 5	Phase 6	Post peak	Possible new wave	Post-pandemic
Uncertain probability of pandemic			Medium to high probability	High to certain probability	Pandemic in progress	—	—	—
Animal-to-animal infection only	Animal-to-human infection	Sporadic or clustered cases in humans	—	—	—	—	—	—
—	(Considered a human pandemic threat)	No sustained community-level outbreaks	Sustained community-level outbreaks	Sustained in two countries in one WHO region	Sustained in country in another WHO region	Levels drop below peak in most countries	Activity rising again in most countries	Levels return to ordinary seasonal levels
● Phases 3-6: "Sustained" implies human-to-human transmission. ● After Phase 6: "countries" implies those "with adequate surveillance". ● WHO no longer officially uses "pandemic" category.

In a virtual press conference in May 2009 on the influenza pandemic, Dr. Keiji Fukuda, Assistant Director-General ad interim for Health Security 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 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, revised in 2005 and 2009, defining 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 definitions of a pandemic and the phases leading to its declaration, were 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.

Severityedit

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 on healthcare services

To account for the limitations of measuring the case fatality rate alone, the PSAF rates severity of a disease outbreak on two dimensions: clinical severity of illness in infected persons; and the transmissibility of the infection in the population. Each dimension can be measured using more than one metric, which are scaled to allow comparison of the different metrics. Clinical severity can instead be measured, for example, as the ratio of deaths to hospitalizations or using genetic markers of virulence. Transmissibility can be measured, for example, as the basic reproduction number R₀ and serial interval or via underlying population immunity. The framework gives guidelines for scaling the various measures and examples of assessing past pandemics using the framework.