07 December 2021

Omicron evolution

It's too early to say for certain, and the assessment could still change as new data become available, but first indications suggest that the new omicron variant, though extremely transmissible, may cause less severe illness than earlier variants of covid-19 (more here).  That's not to say it's harmless.  Some omicron cases in South Africa have required hospitalization.  But in fully-vaccinated people the symptoms are usually mild, and I have still not seen any reports of death resulting from omicron infection, even in the unvaccinated.

If this turns out to be the case, we should not be surprised.  Higher infectiousness but less-severe disease are exactly what one would expect natural selection to produce.

In a disease microorganism, it's easy to see why natural selection would favor greater infectiousness.  A variant form which spreads more easily will, in fact, spread more, and will out-compete less-infectious variants to become predominant in the population.  But selection will also favor milder disease.

Imagine a virus producing such severe disease that it kills a person within just five minutes of being infected.  Such a virus would be an evolutionary failure.  In five minutes, it's unlikely that the infected person would get a chance to pass it on to other people, and so it would die with its first victim.

Now imagine a mutated form of this virus, a variant producing milder disease, taking five days to kill instead of five minutes.  That variant would have far more opportunities to be transmitted before the first host dies, and so would easily out-compete and replace the original form.  A further mutated variant producing even less severe disease, which doesn't kill the victim at all, would be even more successful, since it could be transmitted for however long it remained in the victim's body until the immune system cleared it out.

In fact, from the viewpoint of a virus's evolutionary success in a human population, the optimum level of disease for it to produce is zero -- that is, to not make people sick at all.  Humans understand how infectious diseases work and can take steps to minimize contact with individuals who are clearly ill.  So all other things being equal, a virus variant which produces no illness at all will spread more easily than more damaging variants, and out-compete them for survival.

So the evolution of viruses and bacteria naturally tends to mitigate the severity of the diseases they cause.  There are several diseases, such as syphilis, for which historical records show that their symptoms a few centuries ago were substantially more severe than they are today.

The delta variant partly deviated from this pattern in being more infectious than the original covid-19 but causing more severe illness.  Even so, it is so much more infectious that it out-competed and largely replaced the original version despite its greater propensity to kill its victims.  But the latter really was a liability from the viewpoint of virus survival, even if a surmountable one.

It's logical that if a new variant appears which is even more infectious and causes less-severe disease, it will out-compete and eventually replace delta, a further new stage in covid-19's evolution.  Omicron may turn out to be that variant.  If not, eventually some such variant will appear and become dominant.  Given enough time, we can expect successive mutations to steadily reduce the danger covid-19 poses.

This doesn't mean we just have to wait for omicron to displace delta and our troubles are over.  A highly-infectious disease that puts some fraction of its victims in the hospital is still a serious problem, even if it rarely kills people.  But it's worth remembering that in the long run natural selection will work in our favor.


Blogger Sixpence Notthewiser said...

The way you put it, it all makes sense: highly infectious but not deadly has a higher chance to stay in the population longer. Still, these mutations are not fun. I've watched politicians dismiss this 'new variant' as a ploy because some elections are coming. And part of the population responds by still refusing to be vaccinated or even wear masks. It blows my mind.
I was talking to a friend, and he said that most probably we'll have COVID for the long run, given that some of the population don't even think it'll get to them I think I agree...


07 December, 2021 04:27  
Blogger Lady M said...

The corona virus does seem to be mutating to be more like the flu or the common cold. It will still kill the very old, sick and frail like influenza does but will not be as lethal to the population at large. Influenza viruses and cold viruses are very successful. So Covid will be with us forever if it mutates that way. As for me, it has taught me that wearing a mask, scrubbing my hands, and avoiding humans are great ideas. I have not caught a single contagious disease in the last 2 years and I think I will make this my normal operating procedure. Also keeping up my immune system with fiber rich healthy food for my gut bacteria will add additional protection.

07 December, 2021 05:32  
Blogger Infidel753 said...

Sixpence: The wingnuts will politicize this to the bitter end and insist that every new development, bad or good, is part of a conspiracy. But biology will do what it does, and so will science.

I tend to agree that covid-19 won't disappear for the foreseeable future. It will probably become like the flu -- an endemic disease with a steady but low death toll, for which we'll need to get a vaccine shot every year or six months (and some people will keep refusing the shots and keep being more likely to get sick, as with the flu). Once it's no worse than that, we'll probably end up just living with it. The big psychological point will come when masks aren't needed any more. People will never accept masks as a permanent norm -- being able to see facial expressions is too important a part of normal social interaction.

07 December, 2021 05:36  
Blogger Infidel753 said...

Lady M: I've had the same experience -- much healthier since the pandemic began, since the measures to avoid covid-19 help avoid other airborne diseases as well. And it was part of what helped motivate me to start eating healthier last year.

In all honesty I won't mourn if covid-19 keeps pruning back the wingnut / anti-vax population a bit, though.

07 December, 2021 05:44  
Blogger Mary said...

Your last sentence to Lady M is exactly how I feel.

07 December, 2021 07:26  
Blogger NickM said...

I dunno about the masks... For me it isn't the facial expressions as much as the steamed-up spectacles that is the real pain. I am surely not alone here.

Whilst, obviously, evolution will usually tend to favour spread over lethality (which is of no more "use" to the virus than it is to the victim*) this is not always the case. HIV is a striking counter example. It generally takes years from infection before any serious symptoms kick in. During all that time the host is infective. Eventually it is almost invariably fatal** though.

*You wanna get a microbiologist in a tizzy - ask them if a virus is alive.
**There appear to be a vanishingly small number of people who spontaneously become HIV-.

07 December, 2021 09:05  
Blogger Infidel753 said...

Mary: I think a lot of people feel that way at this point.

NickM: HIV kills so slowly in most cases that its lethality barely has any effect on spread, so there's not much for natural selection to work with. Even so, in the absence of the drugs we now use to extend the lives of infected people (and reduce infectiousness), I'd expect that less-lethal variants would spread more successfully and eventually become predominant. It would just be a very small difference in success and thus a very slow change.

07 December, 2021 10:26  
Blogger Mike said...

And I agree with Mary.

07 December, 2021 10:28  
Blogger SickoRicko said...

Thank you very much for this terrific explanation! It makes so much sense.

08 December, 2021 10:56  
Blogger yellowdoggranny said...

as long as everyone doesn't get vaccinated we're fucked

08 December, 2021 13:59  
Blogger Kwark said...

Soo. . . You're saying that evolution is real. Just to start, that's probably a no go for a large percentage of the antivax Q crowd.

09 December, 2021 10:40  
Blogger Infidel753 said...

Mike: I think they'll keep granting our wish.

Ricko: Thanks! I do always try to be clear.

Granny: Not necessarily -- see post.

Kwark: Well, evolution happens whether they believe in it or not.

09 December, 2021 19:10  
Blogger Al Penwasser said...

Outstanding write-up!

09 December, 2021 19:34  
Blogger Infidel753 said...


09 December, 2021 20:14  
Blogger Bohemian said...

Yes, Evolution of Parasitic Organisms generally favor them not killing off their Hosts to be the most successful.

10 December, 2021 22:03  
Blogger Infidel753 said...

Given enough time, most diseases would probably become pretty much harmless. Unfortunately that would take far more time than we could afford to allow it.

11 December, 2021 00:46  

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