SARS-Cov2 has a fatality rate of maybe 1-2%. I think the original SARS from 20 years ago is closer to 10-15%, and MERS is over 30% fatality.
So from an evolutionary POV, whatever disease is the most contagious is the one that succeeds. Alpha displaced the original strain, then delta displaced alpha. It was my understanding in disease evolution that there was generally a tradeoff in infectivity and seriousness of the disease. If people aren’t able to be up and walking around, the disease doesn’t spread as much (diseases spread by vectors like mosquitos are not held to this same rule, I don’t think).
But with SARS-Cov2 you have the infectious period where you’re maybe asymptomatic or just have a cold, then the disease migrates to the lower lungs and causes more serious issues.
From an evolutionary point of view, is there any advantage for SARS-Cov2 to evolve to be both as infectious as delta but as fatal as the original SARS or MERS diseases? Would that provide any evolutionary benefit over the current delta variant and would the natural pace of evolution lead towards this direction?
Or does the fact that the virus migrates to the lower lungs after you’ve already spread it to others mean that there isn’t much pressure for the virus to cause more serious lower lung infections?
There’s no inherent evolutionary advantage for a pathogen to kill its host. Ultimately evolution favors transmissibility- how successfully a pathogen perpetuates itself. Lethality is essentially a side effect of an organism being highly efficient at reproducing at the host’s expense, but this doesn’t count in the long run if it can’t spread to new hosts.
The most lethal infections combine very rapid reproduction with very easy transmissibility. Where sanitation is poor cholera evolves to create severe diarrhea. Malaria relies on ubiquitous mosquito vectors, and cold/flu on respiratory infections in crowded indoor conditions.
Smallpox (V. major) had that profile and did very, very well for thousands of years. Maybe even longer, but written records of it only go back so far.
Of course, we humans eventually got to the point we learned how to exterminate it, and we did. It might have been “advantageous” for a long while… until suddenly it wasn’t any more.
The only thing that is “advantageous” from the standpoint of evolution is what allows continued reproduction - whether that’s low lethality or high, easily spread or not, it doesn’t matter as long as the disease continues on generation after generation. Who leaves the most kids/grandkids/great-grandkids/etc. is the only measure of “success”.
Ideally, any organism that depends on a host to survive wants its host to live long enough to be able to replicate itself and move on to another host. If it can do that and kill its hosts in the process, so be it, from the viruses perspective, but that’s just a side effect. With respiratory infections such as SARS, COVID-19, and influenza, what typically (and ironically) ends up killing us isn’t the virus itself but an errant reaction by the immune system.
Lethality of any sort is never in a virus’ interest, as pointed out abovethread. The less lethal a virus is, the better for it - all that matters is contagiousness. The best virus, survivally speaking, would be one that is extremely contagious but does no harm to its host at all.
It’s not like Covid is only contagious before it moves deep into the lungs. Someone with infected lungs is contagious, too, as is anyone else whose body has not yet completely cleared the infection. And so it’s to the virus’s advantage for all of those folks with deep lung infections to keep on living so they can keep on spreading it.
Right- anything that would make the host stay inside and not be out and about transmitting it tends to work against spread. Killing the host is kind of the ultimate end point for that.
This number is way too high for the majority of the population. Risk of death is extremely low for younger people and increases exponentially with age.
Infection fatality rate per the CDC:
0-17 years old: 0.002%
18-49: 0.05%
50-64: 0.6%
65+: 9%
I’m not able to find a rolling infection fatality rate (deaths/all infections) chart, but the case fatality rate (deaths/confirmed infections) is the lowest now in the US that it has been since the beginning of the pandemic.
I’ve not seen anything showing Delta to be more deadly than previous variants. Also, much of the vulnerable population has been vaccinated by now, so we should expect to see a lower death rate, ceteris paribus.
Having said that, it’s still very possible for this virus to become several times more lethal and extremely efficient in spreading. If we think of the virus as a sentient being (which it’s not obviously), then the danger the virus faces in becoming too lethal is that it scares the bejesus out of everyone to the point where they minimize in-person contact, giving the virus limited opportunities to spread. In reality, though, there will always be people out and about, willing to risk it even knowing that death could be lurking around the corner. Look at how many people haven’t even bothered to get vaccinated. It is still very possible we could end up with a nightmare variant. Humans don’t typically think about what might happen; our brains tend to think in the ‘here and now’.