AMS wrote: ↑
Sat Mar 21, 2020 7:20 pm
I think I've heard the UK's chief science advisor comment that viruses tend to evolve to become less virulent over time, and I've been wondering about this. SARS-cov-2 certainly has its opportunity to evolve in the coming millions of cases.
With some back of a fag packet level reasoning, we have a virus that effectively transmits person to person, and importantly can be asymptomatic in many (possibly the majority of?) cases. On top of that, long term we will surely be planning to screen good and hard for this one, because governments world wide are going to want to quickly stamp on any flare ups. Is this enough to create a selection pressure on the virus to become "more asymptomatic", which it is clearly capable of being? Some of the data shows it is able to replicate independently in the throat and lungs, and it's the latter that causes the problems. A strain that loses the lung effects but retains the ability to transmit as a throat infection could have a big selection advantage, and one day become the dominant variant.
Does that make sense or is it just overoptimism?
I think the trick is to be asymptomatic for as long as possible, transmit as much as possible, and ideally, not kill your host (because the longer your host lives, the more you can transmit).
However, the chief science advisor might, possibly, be working with outmoded thoughts:
PLoS Pathog. 2014 Oct; 10(10): e1004387.
Published online 2014 Oct 23. doi: 10.1371/journal.ppat.1004387
Theory and Empiricism in Virulence Evolution
Early theories of virulence suggested that pathogens would evolve to avirulent commensals since harming the host would be a poor long-term survival strategy. This view was challenged in the mid-20th century as evolutionary biologists and population geneticists considered how competition among different strains of a given pathogen would influence the evolution of virulence (see  for an excellent historical review).
 J Evol Biol. 2009 Feb;22(2):245-59. doi: 10.1111/j.1420-9101.2008.01658.x.
Virulence evolution and the trade-off hypothesis: history, current state of affairs and the future.
It has been more than two decades since the formulation of the so-called 'trade-off' hypothesis as an alternative to the then commonly accepted idea that parasites should always evolve towards avirulence (the 'avirulence hypothesis'). The trade-off hypothesis states that virulence is an unavoidable consequence of parasite transmission; however, since the 1990s, this hypothesis has been increasingly challenged. We discuss the history of the study of virulence evolution and the development of theories towards the trade-off hypothesis in order to illustrate the context of the debate.
And while not immediately relevant to the above, in my recent reading around, I was impressed by pneumonic plague. Pneumonic plague (which is bacterial) is nasty because it suppresses the immune system[1
] and [2
] while replicating like mad, so when you do finally start coughing, you are highly contagious. You tend to collapse and die pretty soon afterwards, though. This tends to get noticed, so encouraging people to adopt effective
isolation tactics usually works. However, plague has swept through human populations more than once.