Delta undoubtedly has many advantages, and disadvantages, over earlier variants that it outcompeted. That's why I said viral load is "a factor".
One thing that's interesting to me is how the selective pressures on pathogens change. Early on, adaptations that cause higher viral loads are often highly advantageous because they aid transmission-- this tends to be correlated with higher virulence, too. Later, as a smaller portion of the population is susceptible, and adaptations that are less virulent, have longer incubation periods, etc, can win.
It'll be interesting to learn about Omicron virulence and transmissibility. I wouldn't be surprised if it's less, like early reports hint at. However, disentangling this from partial immune escape, etc, is hard.
> To better understand the coronavirus’s journey from one person to another, a team of 50 scientists has for the first time created an atomic simulation of the coronavirus nestled in a tiny airborne drop of water.
> To create the model, the researchers needed one of the world’s biggest supercomputers to assemble 1.3 billion atoms and track all their movements down to less than a millionth of a second. This computational tour de force is offering an unprecedented glimpse at how the virus survives in the open air as it spreads to a new host.
And they look at things like how mucins, water, and calcium atoms interact with the proteins of the virus particle.
> This discovery may help explain how the Delta variant became so widespread. Delta’s spike proteins have a more positive charge than those on earlier forms of the coronavirus. As a result, mucins huddle more closely around them. That attraction could potentially make the mucins a better shield.