2. In principle, the virus could mutate in any number of ways, but two of the most important to think about involve VIRULENCE: the amount of harm the disease causes to its host, including mortality risk, and TRANSMISSIBILITY: the ease with which new infections are generated.
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3. Let's start with transmissibility. Natural selection will favor variants of the virus that are more likely to transmit and initiate new infections. For example, a variant might reproduce better in the upper respiratory tract and thus be spread more in respiratory droplets.
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4. If such variants arise, we would expect them to spread, because they would produce more new infections than the previous "ancestral" strains. We certainly can't rule out the possibility of this happening, but I don't there is good evidence that it has happened yet.
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5. It's hard to predict for certain, but my personal opinion is that given the rates at which we seeing mutations in the virus and the structure of its genome, there is not strong reason to expect more transmissible variants to evolve and become widespread over the next year.
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6. Now let's consider virulence. One argument has it that diseases should evolve not to kill their hosts too quickly, because a dead host doesn't transmit to other people. There are a lot of problems with this argument both in general and for
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7. People used to take this reasoning to the conclusion that microbial pathogens should co-evolve with their hosts to cause minimal harm. The problem with that argument is that there tend to be *virulence-transmissibility tradeoffs*.
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8. To transmit effectively, a pathogen generally needs to make a lot of copies of itself within the host. Doing this tends to cause harm to the host, either directly or as a byproduct of the host's immune response. Of course there are exceptions. Biology always has exceptions.
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9. So we might expect a pathogen to evolve intermediate virulence. Paul Ewald has proposed this as a reason why we shouldn't worry too much about H5N1 bird flu. It may now kill over 60% of people infected, but if it takes off in humans shouldn't it evolve to become less lethal?
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10. I've been very critical of this line of thinking: Perhaps in the long run, a pandemic H5N1 would decline in virulence. But to misuse John Maynard Keynes, in the long run we are all dead. Especially in a bird flu pandemic. Beneficent evolution would be little consolation.pic.twitter.com/UZhPbe5H46
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11. More generally, predicting virulence evolution is hard, especially for an emerging disease. When a new pathogen enters the human population, it is unlikely to be adapted to maximize transmission. It might be too virulent, or too mild.
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At this point in history, I expect that humanity's response to a pathogen is an important factor in its evolution. If COVID-19 was more lethal, would travel restrictions not have been happening earlier and with greater decisiveness? It may already be too virulent for its own good
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Replying to @Plinz
Well, its virulence affects the *epidemic trajectory* via human responses. But unless it evolves into multiple strains, some of which we tolerate and others of which we fight aggressively, natural selection isn't acting on virulence via human responses.
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