There are a variety of hypothesis as to why flu rates are so low this year. From Joseph Mercola at lewrockwell.com:
With COVID-19 still dominating headlines, influenza (flu) has been conspicuous in its absence, especially during what is typically peak flu season. The U.S. Centers for Disease Control and Prevention (CDC) tracks influenza (flu) and pneumonia deaths weekly through the National Center for Health Statistics (NCHS) Mortality Reporting System.
It also creates a preliminary estimate of the burden of seasonal flu, based on crude rates of lab-confirmed flu hospitalizations. Such estimates are intended to give an idea of how many people have been sick from or died from the flu in any given season — that is, except for 2020.
“April 4, 2020, was the last week in-season preliminary burden estimates were provided,” the CDC wrote on its 2019-2020 U.S. flu season webpage.1 The reason the estimates stopped in April is because flu cases plummeted so low that they’re hardly worth tracking. In an update posted December 3, 2020, the CDC stated:2
“The model used to generate influenza in-season preliminary burden estimates uses current season flu hospitalization data. Reported flu hospitalizations are too low at this time to generate an estimate.”