The thing about herd immunity is that we haven't conducted the spirulogical tests on a mass scale to identify how much of the population actually has the markers of immunity after getting infected and recovering.

We know that people were infected in Nov and that they were travelling all around the world infecting other people. Now if any of the estimates of r0 are remotely accurate, who knows how many people have already been infected. The actual infection rates of most countries could be much much higher than we think. Probably a huge percent of people have already had it and were fine.

Maybe what you thought was the flu 3 months ago was actually COVID-19.