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 influenza risk assessments to improve preparedness and inform proportional response actions (Pandemic Influenza Risk Management guidance (PIRM)).

Pandemic Influenza Severity Assessments can also be used to improve the interpretation, visualization, communication of influenza data during seasonal epidemics.

Influenza severity is defined in terms of four indicators: transmissibility of an influenza virus, seriousness of influenza disease, morbidity and mortality and the impact on healthcare capacity.

Main changes in the updated guidance below, compared to the first edition published in 2017:

• Splitting of the previous indicator "impact" into two indicators, "morbidity and mortality" and "impact on healthcare capacity"
• Option to report syndromic and/or influenza-specific assessments
• Development of publicly accessible global outputs by WHO