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Respiratory viruses tend to evolve to be more transmissible but less virulent

This blog post was adapted from this Twitter thread:

Articles often claim that the Delta variant is more virulent, e.g. “Citing the spread of the more virulent Delta coronavirus variant in the United Kingdom”. Earlier in the year the same was said about the Alpha (Kent) variant, that it was more “virulent”.

That was untrue. Virulent means “harmful”, not “infectious”.

If anything, the evidence suggests that the Delta variant may be less virulent, but more transmissible/infectious—although it is hard to be sure this is true, given that the vulnerable old are now protected by vaccines.

“The suggestion that the Indian variant is more pathogenic needs to be taken with a big dose of salt. The same was initially suggested for the Kent variant but was later shown not to be true” writes Professor Ian Jones.

Respiratory viruses tend to evolve to be more transmissible but less virulent: they do better if you go out and about meeting people. This is not true of insect-borne or water-borne viruses, which don’t care how sick you are: insects or water do the going out about about for you.

Photo by Edward Howell on Unsplash

That’s why there are 200 kinds of common cold, none of which is virulent—including OC43, a coronavirus that probably caused a terrible pandemic in 1889 but is now a fairly mild and common cold:

But there is one situation in which respiratory viruses will stay virulent or even become more so. That’s if mild cases meet fewer people than severe ones.

This appears to be what happened in 1918, Paul Ewald argued, when severe flu cases were evacuated from the front line to a series of staging posts, field hospitals and trains, while mild cases slept it off in a dugout. And I think it is what happened last year when mild cases of Covid-19 stayed locked down at home while severe cases went to hospital, where many cases of the virus were acquired in hospital.

In other words, lockdowns (whether necessary or not) probably delayed the evolution of the virus into a milder form. That is now happening, and is our least worst option given that eradication is impossible and the virus may become more transmissible in response to vaccination.

In general, there is not nearly enough thinking along the lines of “Darwinian medicine” within the medical establishment, as Randy Nesse has long argued.

Repeat 100 times: Evolution is not mutation. It is mutation and selection.

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By Matt Ridley | Tagged:  coronavirus