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Measuring or estimating the relative severity of an influenza virus is difficult because an influenza virus that has a very high case fatality rate (CFR), but a very low transmission or attack rate may cause fewer deaths than a virus that has a moderate to low CFR, but very high transmission or attack rate.  From the public health perspective, the relative severity of an influenza virus is best estimated by the number or rate of influenza deaths in a defined population, i.e., the influenza mortality rate (IMR). 

To calculate or estimate the CFR for an influenza pandemic, CDC has arbitrarily assumed that in all influenza pandemics, 30% of the total population have an influenza-like-illness (ILI) and this number is used by CDC as the denominator of influenza cases to calculate the influenza case fatality rate (CFR).