Influenza

Pandemic Influenza Severity Assessment (PISA)

National Museum of Health and Medicine, Armed Forces Institute of Pathology, Washington, DC. Image number NCP 1603

In 2011, the IHR Review Committee on Pandemic Influenza (H1N1) 2009 recommended WHO develop and apply measures that can be used to assess the severity of every influenza epidemic:

"By applying, evaluating and refining tools to measure severity every year, WHO and Member States can be better prepared to assess severity in the next pandemic."

The Pandemic Influenza Severity Assessment is intended for use by Member States (MS) and WHO as part of their overall pandemic risk assessments (Pandemic Influenza Risk Management guidance (PIRM)).

Influenza (pandemic) severity is defined in terms of three indicators: transmissibility of an influenza virus, seriousness of influenza disease and impact.


The aims of the influenza severity assessment are to:

  • describe the epidemiological situation and assess the severity of an influenza epidemic or pandemic based on all available information,
  • inform national and global risk assessments, and
  • inform public health preparedness, response and recovery measures as well as resource allocation.

For more detailed information on PISA, please see the links below:

Contact us at WHOinfluenza@who.int