The [Silverman and Washburne] paper reckons that 7m Americans were infected from March 8th to 14th

Silverman and Washburne reportedly gleaned their data from weekly reports by 2,600 American doctors on the number of their patients who have ili

The authors assume that the share of these providers’ patients with ili who do have the flu matches the rate of flu tests that are positive in the same state and week. This lets them estimate how many people have ili seriously enough to call a doctor, but do not have the flu—and how many more people have had non-flu ili in 2020 than in prior years.

Interesting approach. We should know for sure in the next few weeks as they start large scale antibody testing.