Viruses don’t mutate to be less deadly; they mutate to be more contagious

So, I read this article, “Omicron possibly more infectious because it shares genetic code with common cold coronavirus, study says”  today, and thought…”interesting…” 

Sure, we shouldn’t expect more than a preprint at this point, but, seems like the journalist could at least find some voices who might bring some reasonable skepticism of the flashy claims.  But… nope. What really pulled me up short was this:

As a virus evolves to become more transmissible, it generally “loses” traits that are likely to cause severe symptoms, Soundararajan said. But he noted that much more data and analysis of omicron was needed before a definitive determination could be made, adding that unequal distribution of vaccines globally could lead to further mutations of the coronavirus.

Was pretty damn sure I definitely had read otherwise on multiple occasions from two of the best epidemiologists on twitter, and indeed…

You should read the whole thread from Bergstrom, but here’s the key part:

As for that Post article, my virologist friend had this to say, “Not impossible, but they are basing their conclusions on 3 amino acids. It gross over interpretation of the data.”

But, anyway, on the larger point.  There is, obviously, huge selection pressure to become more contagious.  That’s absolutely natural selection in action.  But, for the most part, there’s no selection pressure to be more or less deadly.  What happens there, as I understand it, is almost just an unintended side effect one way or the other from evolving to be more contagious.  A super-deadly disease has reason to evolve to be less deadly as it can burn out and lose hosts to spread.  But for a disease with an infection fatality rate in the low single digits at best, there’s just no evolutionary pressure to evolve to be less deadly.  Now, again, it just might happen– and that would be awesome– but there’s no reason to expect that it will.  Though interestingly, a lot of people in the medical field seem to not understand this really key point.  


About Steve Greene
Professor of Political Science at NC State

One Response to Viruses don’t mutate to be less deadly; they mutate to be more contagious

  1. itchy says:

    I’d say it’s extrapolating too much to simply assume omicron will be less deadly. But not that the virus will eventually be less deadly.

    “there’s just no evolutionary pressure to evolve to be less deadly.”

    There doesn’t have to be if a random mutation would more likely make a virus less deadly, and that’s a good bet.

    Richard Dawkins once said, “However many ways there may be of being alive, it is certain that there are vastly more ways of being dead.”

    Which means any random mutation is more likely to make any quasi-organism less effective at doing its thing, rather than more effective. Over time, entropy is a strong force.

    Yes, a single mutation might be more likely to be neutral. But when you stack mutation after mutation the overall effect points strongly toward the negative.

    (This is why we avoid radioactivity-induced mutations in ourselves. Sure, we could become Spiderman, but chances are much greater we’ll have cancer.)

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