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From a TV news report of a press conference from (I think) the German Robert Koch Institute, I remember hearing an expert declare that he was expecting COVID-19 to get less deadly over time. Unfortunately, I don't know what press conference it was (I'm not sure if it was the Robert Koch Institute — it might also have been the Charité university hospital) and cannot find such a stated expectation on the internet.

Why would an expert express this expectation?

Is it because we're heading toward summer (at least where most COVID-19 cases have been reported), or is it a standard pattern, perhaps either due to people's immune systems or due to the virus evolving and becoming weaker?

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  • $\begingroup$ Comments are not for extended discussion; this conversation has been moved to chat. $\endgroup$
    – Bryan Krause
    Commented Apr 20, 2020 at 15:20

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While the data are much too sparse and noisy to give an answer about what is happening to COVID-19's virulence (the technical term for the "deadliness" of an infectious disease), or to forecast what will happen to its virulence in the future, there are indeed theoretical reasons that one might expect the virulence to decline in the future.

There is an entire theoretical literature on the evolution of virulence (e.g. see Read 1994, or see Wikipedia); Robert May and Paul Ewald are two researchers who contributed seminal ideas. The basic idea is that infectious 'organisms' (including viruses) may face a tradeoff between their ability to produce lots of infectious particles (e.g. by replicating faster within the host) and the length of time that they can stay in the host before it dies or activates immune defenses sufficiently to end the infection. In this case, some intermediate level of virulence may evolve.

Evolution of virulence in ways predicted by the theory has been observed in the real world in such diseases as myxomatosis (a viral disease of rabbits). The theory has also been suggested to apply in HIV and syphilis.

The actual forecast can depend on many details of the biology and epidemiology of a particular organism. If COVID-19 happened to have a higher-than-optimal virulence in humans at the time of emergence, we could expect its virulence to decline over time. It could also decline if epidemic control was applied in such as way as to differentially affect more-virulent strains; this has been suggested by Tang et al. on the basis of the pattern of mutations in different strains, although their logic has also been criticized on Twitter by an expert in genetic epidemiology, and now by a more official rebuttal by McLean et al. (with some back-and-forth discussion between the authors and critics).

This explanation doesn't include the possibility of host evolution, which is theoretically important but not really relevant to COVID-19 — it would only happen over many generations, and only if the epidemic was so severe that it was a significant overall cause of death or failure to reproduce ...


  • Tang et al. "On the origin and continuing evolution of SARS-CoV-2", National Science Review, nwaa036, https://doi.org/10.1093/nsr/nwaa036 .
  • Read, Andrew F. "The evolution of virulence." Trends in microbiology 2.3 (1994): 73-76.
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  • $\begingroup$ To summarize, the most performant virus is the one that keeps the hosts well enough and socializing for the most time with the highest output of virus to others in their vicinity. $\endgroup$ Commented Dec 29, 2022 at 9:20
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There's a theory that a highly deadly virus kills its hosts and therefore replicates less. With Covid killing somewhere between 0.1% and 1% of infected people, that seems to be completely irrelevant.

There's a better theory that when a virus is very deadly then people will do whatever they can to stay away. Boris Johnson wouldn't have shaken hands with Ebola patients. A virus same as Covid but 50% lethal would have massive voluntary lockdown and fights for vaccines. For those reasons, it would spread less than Covid. With 50% lethality you could expect 99% of people to lock themselves up out of fear, and the 1% in the streets shot on sight.

I am told that the current Omicron variant has changes which body parts it affects (throat instead of lungs). And these changes make it at the same time less lethal and more infectious. This sounds reasonable, but not necessarily true. So you may try to think of something that makes the virus both more lethal and more infectious at the same time.

But fact is that the virus has no advantage from killing people. No evolutionary gain. However, it has an advantage from being more infectious. On the other hand, Covid is very infectious already, so a random change is very unlikely to make it more infectious. The "better" it is at infecting people, the harder it is to improve.

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