system
Given the following post, respond as if you're commenting on Reddit:
human
Why COVID-19 variants mutate so quickly towards higher transmissibility but same thing is not happening with flu?

Original COVID-19 variant reproduction rate was estimated to be around 2.5-3. Now we deal with the variant twice as transmissible. I often hear that influenza mutates at a faster rate compared to COVID-19.

When it comes to flu it seems that worst strains ocassionaly get to around 1.8 r0, but mostly it's around 1.3. Why flu doesn't mutate gradually towards more infectious forms? Most infectious strains actually seems to fade and strains that are both less infectious and deadly seems to take off during some seasons, which is kind of counter-intuitive for me.

When it comes to COVID-19 is there any hope that future variants will be less infectious and deadly? Or we are more likely doomed with crazier and crazier mutations?
gpt
So there are two separate things here. There's infectiousness in the absence of immunity, and then there's the impact of pre-existing immunity of the number of people contracting the virus.

Flu has been around for thousands of years. It's well adapted to humans, but at the same time, most of us have some immunity due to prior infections. Occasionally a new strain will emerge that fewer have immunity to, so it is able to infect more people as it spreads.  But then once we gain immunity to that strain, it's back to "normal".

SARS-Cov-2 on the other hand is a new virus in humans.  First infections were around December 2019 - at the time it's clear it was of very low infectiousness, barely able to pass between humans. It needed to adapt to its new host. Since then it has become better and better at spreading among humans and that process may not yet be complete. Also it hasn't had much in the way of pre-existing immunity to deal with so far, so there are more hosts available for it to infect than for a typical flu epidemic - but that's changing now. In the UK the vast majority of adults now have *some* immunity to it, and it's clear that despite Delta being a more infectious variant, the rate of infections is slowing down (although likely to rise again when remaining restrictions lifted next week).

From now on, the evolutionary pressure on SARS-Cov-2 will be similar to that on Flu - to find ways to combat existing immunity, whether from prior infection or vaccine. Whether it can or not, and what the consequences are for virulence are unknown and all we can do is speculate until the next variant arrives.