Earlier quoted context omitted.
There is literally an entire industrial complex setup to fight the flu on an annual basis, including massive testing & tracking. This is how we have data on annual flu infections. If anything, this year would have been an excellent year to find more flu infections as many people who got sick didn't simply stay home and get over the illness, they went & got tested.
I've never known a doctor actually run a test for flu. You couldn't even get a test for Covid either for much of last year, even if the symptoms were blatant. I know a few people who simply stayed home with Covid symptoms and never ventured out to a doctor - and thus were never counted in the statistics. I had bad flu last year and because of home working, I went back to "work" once I reached the number of days after…
These form the basis of overall infection estimates. If you are interested in the methodology of those estimates, here is the CDC overviews, complete with citations for the primary research that informed the creation of their models: https://www.cdc.gov/flu/about/burden/how-cdc-estimates.htm#e...