Pandemic severity index

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The pandemic severity index (PSI) was a proposed classification scale for reporting the severity of influenza pandemics in the United States. The PSI was accompanied by a set of guidelines intended to help communicate appropriate actions for communities to follow in potential pandemic situations.[1] Released by the United States Department of Health and Human Services (HHS) on February 1, 2007, the PSI was designed to resemble the Saffir-Simpson Hurricane Scale classification scheme.[2][3] The index was replaced by the Pandemic Severity Assessment Framework in 2014, which uses quadrants based on transmissibility and clinical severity rather than a linear scale.[4]


a) Chart showing mortality from the 1918 influenza pandemic in the US and Europe b) emergency military hospital (this was a category 5)

The PSI was developed by the Centers for Disease Control and Prevention (CDC) as a new pandemic influenza planning tool for use by states, communities, businesses and schools, as part of a drive to provide more specific community-level prevention measures.[5] Although designed for domestic implementation, the HHS has not ruled out sharing the index and guidelines with interested international parties.[6]

The index and guidelines were developed by applying principles of epidemiology to data from the history of the last three major flu pandemics and seasonal flu transmission, mathematical models, and input from experts and citizen focus groups.[3] Many "tried and true"[7] practices were combined in a more structured manner:

We also realize as we look back through history is what cities did – 44 cities did, is that many of these measures ultimately every city adopted at some point or another, and the difference may be in the timing of using these measures and whether they're coordinated in an effective way for us to really gain the benefits of them.

— Martin Cetron, Director of CDC's Division of Global Migration and Quarantine[8]


During the onset of a growing pandemic, local communities cannot rely upon widespread availability of antiviral drugs and vaccines[6][9] (See Influenza research). The goal of the index is to provide guidance as to what measures various organizations can enact that will slow down the progression of a pandemic, easing the burden of stress upon community resources while definite solutions, like drugs and vaccines, can be brought to bear on the situation. The CDC expects adoption of the PSI will allow early co-ordinated use of community mitigation measures to affect pandemic progression.[3]


The index focuses less on how likely a disease will spread worldwide – that is, become a pandemic – and more upon how severe the epidemic actually is.[8] The main criterion used to measure pandemic severity will be case-fatality rate (CFR), the percentage of deaths out of the total reported cases of the disease.[3]The actual implementation of PSI alerts was expected to occur after the World Health Organization (WHO) announces phase 6 influenza transmission (human to human) in the United States. This would probably result in immediate announcement of a PSI level 3–4 situation.[3]

The analogy of "category" levels were introduced to provide an understandable connection to hurricane classification schemes, with specific reference to the recent aftermath of Hurricane Katrina.[5][8] Like the Saffir–Simpson Hurricane Scale, the PSI ranges from 1 to 5, with Category 1 pandemics being most mild (equivalent to seasonal flu) and level 5 being reserved for the most severe "worst-case" scenario pandemics (such as the 1918 Spanish flu[3][5]).

A graphical representation of the PSI categorization scheme
CDC Pandemic Severity Index chart[1]
Category Case fatality
1 Less than 0.1% Seasonal flu and 2009 swine flu[10]
2 0.1–0.5% Asian flu and Hong Kong flu
3 0.5–1.0%
4 1.0–2.0%
5 2.0% or higher 1918 influenza pandemic

The report recommends four primary social distancing measures for slowing down a pandemic:

  • Isolation and treatment of people who have suspected or confirmed cases of pandemic influenza
  • Voluntary home quarantine of household contacts of those with suspected or confirmed pandemic influenza
  • Dismissing school classes and closing daycare centers
  • Changing work schedules and canceling large public gatherings[1]

These actions, when implemented, can have an overall effect of reducing the number of new cases of the disease; but they can carry potentially adverse consequences in terms of community and social disruption.[5] The measures should have the most noticeable impact if implemented uniformly by organizations and governments across the US.[5][6][7]


While unveiling the PSI, Dr. Martin Cetron, Director for the Division of Global Migration and Quarantine at the CDC, reported that early feedback to the idea of a pandemic classification scale has been "uniformly positive".[8]

The University of Minnesota's Center for Infectious Disease Research and Policy (CIDRAP) reports that the PSI has been "drawing generally high marks from public health officials and others, but they say the plan spells a massive workload for local planners". One MD praised that the PSI were "a big improvement over the previous guidance"; while historical influenza expert and author John M. Barry was more critical of the PSI, saying not enough emphasis was placed on basic health principles that could have an impact at the community level, adding "I'd feel a lot more comfortable with a lot more research [supporting them]".[7]

During the initial press releases in 2007, the CDC acknowledge that the PSI and the accompanying guidelines were a work in progress and will likely undergo revision in the months following their release.[5]

In 2014, after the 2009 swine flu pandemic, the PSI was replaced by the Pandemic Severity Assessment Framework,[4] which uses quadrants based on transmissibility and clinical severity rather than a linear scale.

See also


  1. 1.0 1.1 1.2 Department of Health & Human Services, (2007) Interim Pre-Pandemic Planning Guidance: Community Strategy for Pandemic Influenza Mitigation in the United States – Early, Targeted, Layered Use of Nonpharmaceutical Interventions Archived February 4, 2007, at the Wayback Machine (Full Text of the Initial Report outlining PSI), US Centers for Disease Control,
  2. Gardner, Amanda (February 1, 2007). "U.S. Health Officials Unveil Flu Pandemic Plan". Washington Post. Archived from the original on November 7, 2019. Retrieved August 21, 2007. U.S. health officials on Thursday outlined an early-warning system similar to that employed for hurricanes to protect and mobilize the country against a flu pandemic.
  3. 3.0 3.1 3.2 3.3 3.4 3.5 Roos, Robert; Lisa Schnirring (February 1, 2007). "HHS ties pandemic mitigation advice to severity". CIDRAP News. Archived from the original on February 5, 2012. Retrieved August 11, 2007.
  4. 4.0 4.1 Holloway, Rachel; Rasmussen, Sonja A.; Zaza, Stephanie; Cox, Nancy J.; Jernigan, Daniel B. (September 26, 2014). "Updated Preparedness and Response Framework for Influenza Pandemics" (PDF). Morbidity and Mortality Weekly Report. Center for Surveillance, Epidemiology, and Laboratory Services, Centers for Disease Control and Prevention. 63 (RR-6): 1–18. ISSN 1057-5987. PMID 25254666. Archived (PDF) from the original on September 20, 2020. Retrieved May 10, 2020. This report provides an update to the 2008 framework to reflect experiences with 2009 H1N1 and recent responses to localized outbreaks of novel influenza A viruses. The revised framework also incorporates the recently developed Influenza Risk Assessment Tool (IRAT) (12) and Pandemic Severity Assessment Framework (PSAF) (13)...PSAF replaces the Pandemic Severity Index as a severity assessment tool (13).
  5. 5.0 5.1 5.2 5.3 5.4 5.5 "HHS unveils two new efforts to advance pandemic flu preparedness" (Press release). Office of Enterprise Communication, Department of Health and Human Services, Centers for Disease Control and Prevention. February 1, 2007. Archived from the original on July 11, 2009. Retrieved August 11, 2007.
  6. 6.0 6.1 6.2 Pellerin, Cheryl (12 February 2007). "Simple Planning Tools Can Help in Early Days of Pandemic". USINFO. Archived from the original on 28 November 2010.
  7. 7.0 7.1 7.2 Roos, Robert; Lisa Schnirring (February 2, 2007). "Experts give qualified praise to new pandemic guidance". CIDRAP News. Archived from the original on May 6, 2013. Retrieved August 11, 2007.
  8. 8.0 8.1 8.2 8.3 "Update of Pandemic Flu Preparedness" (Press release). Office of Enterprise Communication, Department of Health and Human Services, Centers for Disease Control. February 1, 2007. Archived from the original on May 7, 2009. Retrieved August 11, 2007.
  9. Manning, Anita (February 2, 2007). "Government issues pandemic flu plans". USA Today. Archived from the original on May 23, 2011. Retrieved December 7, 2022.
  10. "Keep Calm and Get Vaccinated" (PDF). Centers for Disease Control and Prevention. 2014. Archived (PDF) from the original on August 18, 2022. Retrieved March 22, 2020. Approximately 60.8 million cases and 12,469 deaths occurred in the United States due to the 2009 A/H1N1 Pandemic. According to PSI, which category would this pandemic be classified as? Answer: Category 1.