There’s something in evolutionary biology called the “virulence/transmission trade-off hypothesis”. This says that it can sometimes pay off for a pathogen to be as virulent as possible. You can see this at work when bird flu races through a flock of chickens in a barn, and kills virtually all of them in a few hours. At other times, however, the virus finds it harder to jump from one host to the next. Then, slow-burning strains – strains that cause illnesses that take longer to develop, but make their hosts infectious for longer – will be favored by natural selection. This might be because the potential hosts are spread out and don’t meet very often, or perhaps because a proportion of them are already immune.
I created a simple diagram and model to illustrate this point (click here for further explanation):
We know that viruses that jump a species-barrier can be horribly virulent. They cause illnesses such as bird flu, MERS, SARS-1, SARS-2, Ebola, Lassa and HIV– in fact virtually all of the most virulent current human viruses arrived in our species in the last 100 years. But we can assume that this species-hopping has been going on for millions of years. If something like the trade-off principle hadn’t moderated these bugs pretty effectively, we’d be surrounded by very aggressive pathogens like these all the time.
So we can guess that CoV-2 will eventually become like the other human coronaviruses, or maybe a little worse, like influenza – i.e. something that appears every winter, but doesn’t threaten our way of life. (The intriguing TDVT theory says that the virus in fact curbs its replication using temperature-sensitivity.)
Is there anything that we can do to speed up this gradual process of moderation? Well, yes, almost certainly, there is. We know that different strains of CoV-2 vary in their virulence. A Chinese study (reported in the South China Morning Post) found that the most virulent CoV-2 strains produced 270 times more “daughter” virus particles than the mildest strains. If we can hinder the aggressive strains, the milder ones may spread faster and give rise to immunity before the aggressive ones can arrive.
So track as many people as possible all the time. But focus on testing, tracing and isolating people where someone in the chain has a severe illness. Don’t worry so much about mild illnesses.
The TDVT Hypothesis, as explained by my friend Brian.
For a general discussion of the seasonality of respiratory viruses, written for the layperson, please see: Every winter, colds and flu increase