What happened to the flu?
A disease's R0 (R nought) number measures how easily it spreads.
The flu's R0 is typically around 1.3, while the original COVID
variant's is about 2.8, with Delta coming in at about 5.1, and Omicron
showing indications of being even higher.
This means that COVID is far more transmissible than the flu, so the
modest steps of masks and social distancing that slowed COVID down
shut down the flu almost entirely, when combined with vaccinations.