But doesn't this demonstrate that the selective pressures tend toward higher contagiousness but less severe disease? So if there were a new variant, wouldn't it likely be just more of what makes this one favorable?
I think we aren't very confident yet that there's much selective pressure toward less severe disease - the reason being that the virus does a lot of its spreading before symptoms get to be their worst, so the severity of "worst" doesn't seem to factor into the evolutionary fitness of the virus. And this is supported by the fact that delta was both more contagious and more severe.
We seem to have gotten lucky that omicron is even more contagious but less severe; fingers crossed that bolsters worldwide immunity.
Delta is more infective than Alpha. The mortality rate is comparable if not worse, but it's hard to say as we've had extensive vaccination, which has made it hard to assess. We're lucky for the moment with Omicron, but there is no guarantee.
The design space for diseases are large, and there is no guarantee what happens next. Maybe the next variant is even less lethal, but leaves 90% of people with long term damage. Maybe the next variant is far more lethal, but has a much longer prodromal period with allows it to infect far more people. Maybe we get a variant which is a little less lethal, but persists on fomites for far longer, so it becomes harder to avoid.
The general idea that diseases become less virulent over time is a misinterpretation. Diseases populations and hosts populations co-evolve. Over time, those who are more likely to die from the disease fail to reproduce. Only the most resistant offspring are left in the population.
The rabbit disease myxomatosis killed 90% of rabbits when it was introduced to Australia. Today it kills less than 10% of rabbits. However, myxomatosis is not less virulent.
Lab rabbits have been isolated from natural selection pressure for a very long time. When exposed to today's "mild" myxomatosis the disease still wipes out 90% of them.
The long-term trajectory of our species's co-evolution with covid-19 probably results in fewer of our offspring dying from covid-19 in a hundred years, but that means squat to those of us alive today.
> But doesn't this demonstrate that the selective pressures tend toward higher contagiousness but less severe disease? So if there were a new variant, wouldn't it likely be just more of what makes this one favorable?