I often use the phrase "moderate to severe PanFlu", when I do so I generally use my own criteria for what will constitute severe.
Generally I qualify any CFR at 5% or less as a moderate pandemic, where the CDC has settled on the criteria below 2%.
And just for clarity's sake I hold the opinion that a CFR above 15% would be catastrophic to everyone and everything.
I should probably adjust my phrasing but I won't, I am used to the mental constructs that have informed my thinking up to this point.
Early this year the CDC (Centers for Disease Control and Prevention) developed and published a way to judge pandemics, in the pre-pandemic planning stage, by severity, assigning categories reminiscent of hurricane strength classifications.
The model assumes that 30% of the overall population will get the pandemic strain of the virus, although different age groups and population centers may experience significant differences in the percentages, both overall and in the specific age groups.
Age groups can be thought of as preschoolers, elementary school children, students in upper grades, working adults, older adults, and finally, the elderly.
Official assumptions seem to be that the age group that will experience the highest level of illness is the younger children, picked up in daycare centers and school.
The percentage of the population expected to become ill is termed the Case Attack Rate, sometimes stated as the acronym CAR.
The numbers of those ill who are expected (statistically) to die are the Case Fatality Ratio (CFR).
The pandemic severity index levels are:
- Category 1, CFR of less than 0.1% (example: seasonal flu)
- Category 2, CFR 0.1% to 0.5% (examples: Asian Flu and Hong Kong Flu)
- Category 3, CFR 0.5% to 1%
- Category 4, CFR 1% to 2%
- Category 5, CFR 2% or higher (example: Spanish flu
Recommendations for a response to a flu pandemic were based on the history of the last three major flu pandemics, mathematical models, seasonal flu transmission, and input from experts and citizen focus groups.
It should be noted that the Category 5 on the CDC's pandemic scale has a fatality ratio of 2% or greater. Currently the CFR is just a bit over 60%, both cumulative since 2003, and also if one only includes the cases thus far in 2007. The yearly CFR can, and has, varied from the cumulative CFR, sometimes markedly.
1.8 million deaths in the United States are estimated for a Category 5 pandemic with a CAR of 30% and CFR of 2%. An increase of 1% in the CFR would result in 2.7 million deaths, or an increase of 10% in the CAR would result in 2.4 million deaths.
Each 1/10th of the total US population translates to 30,000 people. Living, breathing, loving, laughing, men, women, and children, not abstract numbers. Something I fear often gets consigned to oblivion as we toss out numbers, levels, categories, and statistics in our presentimental predictions.
There are other variables that drive the severity or mildness of an influenza pandemic, but I will save them for another day.
Finally, while an influenza pandemic based on the officially sanctioned criteria for severe, 30% CAR and 2% CFR would be bad, it could be a great deal worse. At this time no one knows what the next pandemic will be in terms of pathogen, lethality, or numbers likely to be stricken. The range is wide, from barely above "normal" all the way to atrociously catastrophic. We watch, we analyze, we guess; we plan for the worst and hope to have it prove to have been done in vain.