I’m not sure deadliness is orthogonal to reproducibility. You’re correct that the statement you provide is false, but I think I would defend a similar statement as follows:
1. Causing humans to get sick is very likely to make a virus less transmissible, as the host stops moving around as much, or dies. This generally happens in the short term, but if not then in the long term—for example, if a virus transmits solely through relatives touching the corpses of the dead, it may initially be more transmissible the more lethal it is, but once the human populations that maintain this custom have been replaced by those that don’t, then killing its victims rapidly will become a disadvantage.
2. The disadvantage for a virus in causing humans to have symptomatic illnesses in is in tension with the fact that to succeed, viruses need to make human cells stop doing what they’re supposed to do, and start reproducing the virus, which is by definition going to mean our bodies working less well.
3. All viruses face both of these evolutionary pressures. Together they mean that the deadliness of a viral disease in a human population isn’t random but, for a particular virus in a particular population, has an optimal level.
4. When we notice a virus starting to be transmitted between humans, and becoming endemic in the human population, we do so because that virus is more dangerous than all or nearly all other viruses currently in circulation. By virtue of the fact that we have noticed a virus, it is likely that on the “cause less/more sickness” axis it is further towards the “more sickness” end than is optimal.
5. So the new viruses that we are aware of tend to evolve to become less dangerous.
The most obvious weasel words in the above are “for a particular virus in a particular population”. Given that humans evolve, and human customs and immune systems change, in response to viruses, then it could well be the case that in general the effect of viral evolution is dwarfed by the effect of humans evolving, and human societies and immune systems changing, in reaction to the presence of the virus in humans. So viral evolution might not matter much. Even if it does, other evolutionary pressures on the virus, such as avoiding the human immune response, might be far greater than the pressure to become less dangerous to humans. But I would still expect to see new viruses that we are aware of tending to evolve to become less dangerous.
My impression, from reading Ewald’s book Evolution of Infectious Disease long ago, is that this is a better summary than the OP.
We should still be concerned with viral evolution, because it isn’t hard for us to cause harmful evolution. E.g. the 1918 pandemic might be due to evolution on the WWI front, where soldiers who developed the most severe disease were more likely to get transported away from the trenches in ways that enabled them to spread the disease more widely.
This is another field, along with human challenge trials and the vaccination of young children, where the current fixation in medical ethics on not causing harm to an individual might be mistaken. Less clear cut, but still up for discussion. I can think of two areas where slowing viral evolution to greater deadliness might be a policy aim, although in the second alongside the aim of slowing transmission more generally:
1.Should we be using treatments on a small number of the critically ill that are likely to extend their lives, but risk causing the emergence of more deadly variants? I believe there’s a real possibility that variants have emerged as a result of patients being treated with monoclonal antibodies and as a result of lengthy infections in the immunocompromised (I would post links to the papers I’ve just googled, but I’m not sure if the spam filter would let them through and to be honest I can’t evaluate them beyond having heard of the journal they’re published in). If this becomes the consensus, should we be limiting the use of monoclonal antibodies, and reducing any previously prescribed immunosuppressants, in Covid patients, even where this increases the risk of the individual patient dying?
2. In a future epidemic, should it be policy that patients with a serious infection of the novel disease shouldn’t be taken to hospital even where doing so is likely to extend their lives? We’re getting pretty close to nailing up the doors of the sick territory, so surely not unless things are really desperate. But should we have a policy setting out what really desperate means here? And potentially create a list of volunteers who would agree to go to look after the sick in such circumstances, with the understanding that they would be agreeing to isolate with the sick patient in their home throughout the course of the infection and for 30 days afterwards? Or have ambulance staff wear hazmat suits, and transport the potentially infected to repurposed sports halls or other large buildings in the community for basic care by volunteers who agree not to leave the building, with the understanding that these patients are more likely to die than if they went to hospital? A number of countries constructed covid hospitals very rapidly, but I’m not sure anywhere had a policy of moving the infected to places that weren’t hospitals where the volunteer staff would isolate along with the patients.
I’m sure there are other possibilities. As an aside, looking at stuff for this comment I realised that I’d never thought about where the word quarantine comes from. For those similarly ignorant and incurious, quarentena is medieval Venetian for “forty days”, a quarantine period they first imposed as a result of the Black Death.
2. The disadvantage for a virus in causing humans to have symptomatic illnesses in is in tension with the fact that to succeed, viruses need to make human cells stop doing what they’re supposed to do, and start reproducing the virus, which is by definition going to mean our bodies working less well.
I’m ignorant on the topic, but I don’t fully understand this point. I mean, it makes total sense but isn’t covid infectious several days before symptoms appear? This seems to contradict the point. So, is it impossible / or very hard for a virus to reproduce a lot without causing symptoms to its host? Or what is impossible / very hard is “only” to keep the host symptom-free for a prolonged period of time?
And one more, probably stupid, question: could the main symptom of an infection be a general (false) feeling of well-being? For example, by making the hosts segregate specific hormones. That would hide from the hosts the fact that their bodies have troubles, no?
I’m not sure deadliness is orthogonal to reproducibility. You’re correct that the statement you provide is false, but I think I would defend a similar statement as follows:
1. Causing humans to get sick is very likely to make a virus less transmissible, as the host stops moving around as much, or dies. This generally happens in the short term, but if not then in the long term—for example, if a virus transmits solely through relatives touching the corpses of the dead, it may initially be more transmissible the more lethal it is, but once the human populations that maintain this custom have been replaced by those that don’t, then killing its victims rapidly will become a disadvantage.
2. The disadvantage for a virus in causing humans to have symptomatic illnesses in is in tension with the fact that to succeed, viruses need to make human cells stop doing what they’re supposed to do, and start reproducing the virus, which is by definition going to mean our bodies working less well.
3. All viruses face both of these evolutionary pressures. Together they mean that the deadliness of a viral disease in a human population isn’t random but, for a particular virus in a particular population, has an optimal level.
4. When we notice a virus starting to be transmitted between humans, and becoming endemic in the human population, we do so because that virus is more dangerous than all or nearly all other viruses currently in circulation. By virtue of the fact that we have noticed a virus, it is likely that on the “cause less/more sickness” axis it is further towards the “more sickness” end than is optimal.
5. So the new viruses that we are aware of tend to evolve to become less dangerous.
The most obvious weasel words in the above are “for a particular virus in a particular population”. Given that humans evolve, and human customs and immune systems change, in response to viruses, then it could well be the case that in general the effect of viral evolution is dwarfed by the effect of humans evolving, and human societies and immune systems changing, in reaction to the presence of the virus in humans. So viral evolution might not matter much. Even if it does, other evolutionary pressures on the virus, such as avoiding the human immune response, might be far greater than the pressure to become less dangerous to humans. But I would still expect to see new viruses that we are aware of tending to evolve to become less dangerous.
My impression, from reading Ewald’s book Evolution of Infectious Disease long ago, is that this is a better summary than the OP.
We should still be concerned with viral evolution, because it isn’t hard for us to cause harmful evolution. E.g. the 1918 pandemic might be due to evolution on the WWI front, where soldiers who developed the most severe disease were more likely to get transported away from the trenches in ways that enabled them to spread the disease more widely.
This is another field, along with human challenge trials and the vaccination of young children, where the current fixation in medical ethics on not causing harm to an individual might be mistaken. Less clear cut, but still up for discussion. I can think of two areas where slowing viral evolution to greater deadliness might be a policy aim, although in the second alongside the aim of slowing transmission more generally:
1.Should we be using treatments on a small number of the critically ill that are likely to extend their lives, but risk causing the emergence of more deadly variants? I believe there’s a real possibility that variants have emerged as a result of patients being treated with monoclonal antibodies and as a result of lengthy infections in the immunocompromised (I would post links to the papers I’ve just googled, but I’m not sure if the spam filter would let them through and to be honest I can’t evaluate them beyond having heard of the journal they’re published in). If this becomes the consensus, should we be limiting the use of monoclonal antibodies, and reducing any previously prescribed immunosuppressants, in Covid patients, even where this increases the risk of the individual patient dying?
2. In a future epidemic, should it be policy that patients with a serious infection of the novel disease shouldn’t be taken to hospital even where doing so is likely to extend their lives? We’re getting pretty close to nailing up the doors of the sick territory, so surely not unless things are really desperate. But should we have a policy setting out what really desperate means here? And potentially create a list of volunteers who would agree to go to look after the sick in such circumstances, with the understanding that they would be agreeing to isolate with the sick patient in their home throughout the course of the infection and for 30 days afterwards? Or have ambulance staff wear hazmat suits, and transport the potentially infected to repurposed sports halls or other large buildings in the community for basic care by volunteers who agree not to leave the building, with the understanding that these patients are more likely to die than if they went to hospital? A number of countries constructed covid hospitals very rapidly, but I’m not sure anywhere had a policy of moving the infected to places that weren’t hospitals where the volunteer staff would isolate along with the patients.
I’m sure there are other possibilities. As an aside, looking at stuff for this comment I realised that I’d never thought about where the word quarantine comes from. For those similarly ignorant and incurious, quarentena is medieval Venetian for “forty days”, a quarantine period they first imposed as a result of the Black Death.
I’m ignorant on the topic, but I don’t fully understand this point. I mean, it makes total sense but isn’t covid infectious several days before symptoms appear? This seems to contradict the point. So, is it impossible / or very hard for a virus to reproduce a lot without causing symptoms to its host? Or what is impossible / very hard is “only” to keep the host symptom-free for a prolonged period of time?
And one more, probably stupid, question: could the main symptom of an infection be a general (false) feeling of well-being? For example, by making the hosts segregate specific hormones. That would hide from the hosts the fact that their bodies have troubles, no?