I am not sure. Looking at the CDC website [1], it looks like they extrapolate cases for the flu. They seem to estimate that roughly half of the people contaminated with the flu do not see a doctor (I would have guessed the numbers would be even lower, I had the flu a couple of times, it didn't even cross my mind to see a doctor).

[1] https://www.cdc.gov/flu/about/burden/how-cdc-estimates.htm