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I dont believe virus ever increase in lethality as pressures increase, I thought the opposite happens, they naturally becomes less lethal to be able to spread m

I dont believe virus ever increase in lethality as pressures increase, I thought the opposite happens, they naturally becomes less lethal to be able to spread more effectively. Also when you said " stupid to do so" what are you referring to? thx

lamontcg 6y ago
That law doesn't exist. Mostly all the virus cares about is increased transmission, particularly with this virus with a long presymptomatic transmission period, it can evolve to become more lethal if that increases transmission. What would not be selected for would be enhanced virulence that shortens the presymptomatic transmission period. Short term the virus benefits by having higher viral loads and enhanced transmission and enhanced virulence, because it wants those higher viral loads in order to spread more. There's a limit because those higher viral loads are eventually what cause people to feel sick and stay home instead of attend superspreading events, so there's an optimum that its trying to find. As more and more people are immune though, there's probably selection pressure to make the most of whatever superspreading events occur at the expense of non-superspreading transmission. So more people get sick earlier and stay home, but the people who will try to push through a 101F fever the virus wants to have them infecting 100 people instead of 50. Long term the virus is going to evolve to become less lethal though because eventually we're going to hit herd immunity at the point where it can't evolve along the same fitness function to spread epidemically. At that point it needs to make many more mutations to all its epitopes that might be recognized by antibodies and T-cell and achieve enough immune escape to raise its R0 above 1.0 again. There's mutations which slide up and down the existing fitness curve (and some lucky mutations that might make the curve even better) but then there's mutations which are costly which are necessary due to the need for stealth and immune escape. Two different things. Also we have the example of the 1918 pandemic where the later waves were more virulent and lethal than the first wave (but much later it evolved to become seasonal influenza). Got much worse, then got better.
- ionwake 6y ago
  Thank you for taking the time to reply. I don't believe you countered my point. The information in your post was however super interesting and I appreciate that. Why would a virus evolve to be more lethal - if it wanted to increase transmission?

lamontcg 6y ago

That law doesn't exist. Mostly all the virus cares about is increased transmission, particularly with this virus with a long presymptomatic transmission period, it can evolve to become more lethal if that increases transmission. What would not be selected for would be enhanced virulence that shortens the presymptomatic transmission period. Short term the virus benefits by having higher viral loads and enhanced transmission and enhanced virulence, because it wants those higher viral loads in order to spread more. There's a limit because those higher viral loads are eventually what cause people to feel sick and stay home instead of attend superspreading events, so there's an optimum that its trying to find. As more and more people are immune though, there's probably selection pressure to make the most of whatever superspreading events occur at the expense of non-superspreading transmission. So more people get sick earlier and stay home, but the people who will try to push through a 101F fever the virus wants to have them infecting 100 people instead of 50. Long term the virus is going to evolve to become less lethal though because eventually we're going to hit herd immunity at the point where it can't evolve along the same fitness function to spread epidemically. At that point it needs to make many more mutations to all its epitopes that might be recognized by antibodies and T-cell and achieve enough immune escape to raise its R0 above 1.0 again. There's mutations which slide up and down the existing fitness curve (and some lucky mutations that might make the curve even better) but then there's mutations which are costly which are necessary due to the need for stealth and immune escape. Two different things. Also we have the example of the 1918 pandemic where the later waves were more virulent and lethal than the first wave (but much later it evolved to become seasonal influenza). Got much worse, then got better.

ionwake 6y ago
Thank you for taking the time to reply. I don't believe you countered my point. The information in your post was however super interesting and I appreciate that. Why would a virus evolve to be more lethal - if it wanted to increase transmission?