Four Phases of Pandemics

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The World Health Organization (WHO)

Female scientist works in a lab inoculating eggs with flu virus
A microbiologist conducts an experiment in the Biosafety Level 3-Enhanced Laboratory at the CDC. She is inoculating embryonic chicken eggs with the H5N1 influenza virus to help identify genetic markers. Credit: CDC/Taronna Maines.

developed a comprehensive pandemic disease alert system for influenza in 1999. It was revised in 2005, 2009, and again in 2013. The 2013 revision includes a scale that describes a four-phase description of conditions of a pandemic that allows for improved preparedness and response.

According to WHO officials, the new plan focuses on encouraging countries to develop risk management plans. It also focuses more on the disease severity and less on geographic spread, taking into account the variability of conditions in different countries.

The new four-phase alert system includes:

  • Interpandemic—the period between pandemics
  • Alert—a new subtype has been identified and increased vigilance and risk assessment are warranted.
  • Pandemic—a period of global spread of a new sub-type. Global risk assessment based on virology, epidemiology, and clinical data are considered.
  • Transition—a period of decreasing global risk. Stepdowns in response and emergency management occur.

This four-stage system does not include an assessment of how many people are affected or how many die from the disease.  The number of deaths depends not only on the number who are infected, but the severity of the disease (virulence), the vulnerability of affected populations, and the effectiveness of control and containment strategies.