>>However, to conclude COVID-19 is just as lethal as annual flu is a very big mistake, for:

I'm quoting the US CDC which came up with that 0.26% figure based on 25% asymptomatic.

>> We took draconian measures to stop COVID-19, we do not for annual flu. If we let COVID-19 go its course the number of deaths would likely have been an order of magnitude higher.

Agreed, so for better data we should look to the likes of Japan and Sweden that didn't lock down.

However, the weekend's protests without social distancing in locked-down US states and some European capitals, also may assist with estimation of effects if people are crowding together rather than isolating.

>>There still are a lot of unknowns about COVID-19. Are we building up life-time resistance to it once recovered? Will it mutate just like flu and thrive in winter and spring ?

Apparently COVID is very good at mutating, but wrt resistance- all science until now is that whether via vaccine or infection, once you mount the IgG response it's called the "secondary immune" response and I'm not aware of any actual science to cause that longstanding principle to be overturned now, or else there would be no point waiting for a vaccine. Certainly if there's mutation we're not out of the woods by a long shot.