COVID-19: Will It Mutate to Nothingness?

by Raywat Deonandan, PhD
Epidemiologist & Associate Professor
University of Ottawa
(I add my credentials to these COVID-19 blog posts in case they get shared. I want readers to know that my opinion is supposedly an educated and informed one)

My sister recently asked me if viruses like the one that causes COVID-19 always mutate, or evolve, toward less lethality. This is a common question these days since new COVID variants are now appearing regularly, and since COVID misinformation merchant Dr Kelly Victory famously claimed that as viruses mutate, they “become more contagious/transmissible and less lethal.”

This is decidedly not true, as I will try to explain below. But before I do so, here’s my caveat: I am not a virologist or an evolutionary biologist. I’m a lowly global health epidemiologist who observes and studies a variety of diseases and health states on the world stage. In this post, I’m interpreting and translating the work of others.

So… Do viruses always evolve to be less lethal? Some people call this the “Avirulent Theory”. The short answer is “no.”

The thinking behind the Avirulent Theory is that evolution will eventually select for a strain that does not kill the host, because that way the virus survives longer. Remember that mutation is random, and whether a particular mutation thrives depends on environmental factors. It’s not as if the virus is consciously attempting to do something. Rather, “direction” of evolution is an expression of the path of least resistance and greatest opportunity.

Imagine a series of thousands of balls of various sizes being thrown against a wall at random angles and speeds. And imagine there’s a hole of a certain diameter in the wall. If enough balls with enough diversity get launched at that wall, eventually one of them will have the right size, right angle, and right velocity to make it through that hole. The hole “selected” or “filtered” for the right random combination of ball characteristics to be let through. It’s not like that particular ball knew to be a certain diameter and to approach the hole at a certain angle.

Similarly, mutations are random. But if enough mutations occur with enough diversity over enough time, then one will randomly have just the right characteristics to fit well into the environment, and will thrive. And for a virus, thriving usually means that it can find a great many hosts easily.

It’s mathematics. But mathematics is complicated.

It’s tempting to look at that balls-to-the-wall analogy and conclude that any viral mutation will eventually stumble upon a solution that allows it to infect as many people as possible… and that likely means killing as few people as possible. Because if you kill the host, then you can’t infect a lot of people!

But nature has shown us many examples where that is just not the case.

Tuberculosis (a bacterium, not a virus) has been with us for thousands of years and is still just as lethal as it ever was. In fact, we now have XDR-TB, which can evade many of our antibiotics, so it’s more dangerous. Why didn’t it evolve avirulently? Possibly because TB lies dormant in most people and takes a long time to kill you, so its lethality doesn’t compromise its ability to spread.

Rabies (a virus) has been us for thousands of years, as well, but is still close to 100% lethal to humans. Why no avirulence? Possibly because while it quickly kills humans, it can survive in nonhuman reservoirs for a long time. So there’s no evolutionary advantage in not killing humans.

For COVID, in large dense populations, there’s no evolutionary advantage to be gained in not killing the host, because there are plenty of hosts to infect. In more sparse populations, maybe there would be evolutionary pressures to select against lethality.

But COVID (prior to widespread vaccination) kills about 1% of victims. So 99% of the time, it gets to spread unencumbered. It can become considerably more lethal and still not compromise its competitive advantage.

Also, virulence isn’t just about lethality. Virulence is about harm. A virus is free to keep us alive but make us sick and disabled and miserable. But so long as we’re alive, the virus can still spread.

In 1999, West Nile virus mutated into a version that was a lot more lethal (to birds). Similarly, in 2016 Ebola mutated into a scarier strain. And Ebola was already plenty scary!

Sometimes we get lucky and viruses like flu viruses mutate in a less virulent direction, possibly because flu (and other) viruses swap genetic material with other viruses infecting the same host. So many people are already infected with similar viruses that are more benign. So sometimes the first virus gets new genetic code from the old-timer that allows it to be less harmful.

And often what we perceive as “less virulence” is just the result of so many people having got sick that there is sufficient population immunity that the virus has a hard time making many people really sick anymore. That’s probably how the common cold came to be common: each strain started out as a killer centuries ago.

Having said all this, some virologists like Dr Ian Mackay think it’s likely that in a few decades COVID will be reduced to a new strain of the common cold, by virtue of so many global reinfections rendering sufficient residual population immunity.

Bottom line: don’t bank on a particular direction of viral evolution to get us out of this.


(Note: some other good articles on this topic, from which I have stolen liberally, include, “Do Bad Viruses Always Become Good Guys in the End?” and “Fact check: Yes, viruses can mutate to become more deadly“)