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It depends on what you call severity. If severity means the host dies quickly rather than slowly, there's obviously selection pressure and that mutation will d
by fer 5y ago
It depends on what you call severity.
If severity means the host dies quickly rather than slowly, there's obviously selection pressure and that mutation will die off relatively quickly vs the rest, since it won't have as many chances to spread as the less severe strain. E.g. SARS/MERS.
If severity means the host dies more often, but it still takes a long time to die, the selection pressure is much lower, and it will roughly spread the same as the less severe strain, all other things being the same. E.g. Spanish Flu.
- jjk166 5y agoSpanish Flu did evolve to become more mild. All modern seasonal flu variants are descended from it.
- fer 5y agoEventually. But as it appeared it was more virulent than its immediate ancestors.
- jjk166 5y agoEventually in this case being approximately 18-24 months from the first wave, which coincidentally is very close to where we are in the current pandemic.
- bink 5y agoThe Spanish Flu is a single data point. I wouldn't read too much into the 18-24 months stat.
- jjk166 5y agoThe point isn't that Covid will be as fast as the Spanish Flu, it's that the Spanish Flu did not take a long time, and should not be dismissed as an example where it became milder eventually. In fact, we'd have to be quite lucky if covid became mild in the same time frame.
- chimprich 5y ago> Spanish Flu did evolve to become more mild That doesn't make sense. We have seen regular waves of flu, some being more pathogenic than others. There has not been a "direction" of flu strains becoming milder. So-called Spanish flu came after weaker strains. > All modern seasonal flu variants are descended from it. I don't think this makes sense either. The more serious strains of flu we've had in recent decades have for example included contributions from bird and pig viruses.
- jjk166 5y agoCertainly there is variability, evolution is fundamentally a random walk, but no strain since 1918 has come close to that one's severity, and the genes causing cytokine storms in infected individuals, which was why that strain was so deadly, seems to have been quickly lost. Subsequent strains that were highly virulent seem to have evolved new mechanisms. Viruses can pick up genes from lateral gene transfer. Basically in the process of copying their own genetic material while replicating, they might inadvertently copy some extra genetic material that was just lying around in the host cell, and it gets incorporated into their genetic code, producing recombinant strains. Further, just as animal strains can jump into humans, human strains can jump into animals. So for example in 1968 the H3N2 flu lineage started in birds, jumped to humans where it recombined with 1918's H1N1 making human H3N2, and then jumped into pigs to make Porcine H3N2. Indeed there's evidence that H1N1 started out as an avian strain that jumped to humans and swine around the same time, but this is still an open academic debate.