Pandemic Severity Index
Flu Influenza Virus Avian influenza Flu season Research
Vaccine Treatment Genome project H5N1 strain The Pandemic Severity Index (PSI) is 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
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
CDC Pandemic Severity Index
||less than 0.1%
||Seasonal Flu and Swine Flu
||0.1% to 0.5%
||Asian Flu and
Hong Kong Flu
||0.5% to 1%
||1% to 2%
||2% or higher
The PSI was developed by the Centers for Disease Control (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. Although designed for domestic
implementation, the HHS has not ruled out sharing the index and/or
guidelines with interested international parties.
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. Many "tried and true"
practices were combined together 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. Dr Martin
Cetron, Director of CDC’s Division of Global Migration and
During the onset of a growing pandemic, local communities cannot
rely upon widespread availability of antiviral drugs and vaccines
(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 a 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.
The index focuses less on how likely a pandemic is going to get out
of control, and more upon how severe the pandemic actually is. The
main criterion used to measure pandemic severity will be
case-fatality ratio (CFR), the percentage of deaths out of the total
reported cases of the disease.
The actual implementation of PSI alerts is expected to occur after
the World Health Organisation (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
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.
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.