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